Unit buff costs

[Seravy]

Thinking about buff costs a bit more in depth. I think in general, rares should be equally cost effective as others - normally I'd say they should be better, but they provide higher benefits and buffs often have increasing returns - paying 100 for 4 armor is a better deal if 50 grants 2.

Commons
These generally provide a +3 stat increase in one category, or something equivalent to that.  As a common summoned creature costs roughly the same and usually has stats in the 3-4 range, the buffs and the creatures are about equally strong and have the same cost, so this category is good.

Uncommons
Lank Link - cost and effect similar to a common, with slightly better effect/cost ratio.
True Sight - Counters a few specific types of effects (illusion, invisibility). 75 is fair and matches Wraith Form (which provides less useful immunities but additional movement type)
Flight - Similar to True Sight but counters something much more frequent - walking melee units. It also improves movement both in combat and overland. 125 is fair. It would be interesting at 100, as that would allow Magicians to cast it, but not sure if that's a good idea.
Spell Lock - Prevent losing hunderds of mana crystals by protecting the unit or its buffs. Worth the price. (falls into the same group as true sight - situational protection. Cost is similar too.)
Chaos Channels - costs and effect as a common buff, except for flight. 1/3 chance to get something that's worth more than the cost makes it worth the uncommon slot.
Immolation - Free "Fireball" every time the unit fights in melee. Worth 60.
Planar Travel - cheap, no problem
Blood Lust - cost 50, cheap enough. Real "cost" is the unit turning undead. A bit on the overpwered side as is, but that's intended, as it counters normal units specifically.

Rares
Elemental Armor - Like true sight, 75 is fair for near immunity against one type of attack.
Iron Skin - 2.5 times as powerful as Holy Armor, but costs 4 times as much. Overpriced? Considering 120 (still more than 2.5* Holy Armor) to simulate the huge "increasing benefits" effect on armor.
Invisibility - Like Flight and True Sight, this provides protection for the unit. Effective against ranged attacks (all of them), but also reduces melee damage and makes the unit harder to target and find on the overland map. I'd say benefits exceed even "Flight", making this spell worth ~150, so the current cost (175) is not very far off.
Wind Walking - A unique spell. It pretty much does the same as "Flight" or "Wraith Form" on a ship, except this can be used on any unit. That makes it look overpriced, but considering that "any unit" can potentially have several times the movement speed of a ship, I think it's not that far off - maybe 200 would be better?
Magic Immunity - Similar to invisibility, this is still a situational protection that works against a lot of things - in particular all magic ranged attacks (this is weaker than invisibility!) and all combat spells (this is stronger) and gaze/touch attacks (also weaker than Invisibility). Overall it's about as powerful as Invisibility, but costs significantly more. Overpriced? Considering "magic" in general is the theme of the game and immunity to it is a big deal, 25 more than Invisibility is suggested even though the benefit is roughly equal - so a cost of 175.
Invulnerability - Effect is worth roughly as much as 6 armor, but is slightly better (added weapon immunity and not reduced by pierce/illusion), so based on Iron Skin, 150 sounds fair.
Lionheart - Benefits roughly equal to Heroism (on a level 0 unit), but 2.66 times the price. It does have the advantage of being cumulative with unit levels instead of replacing that feature, and it is also better for single figure units (due to the health bonus). Still, seems far overpriced. Problem here is the combat price, which should remain at 40 (as it heals 8 instantly), but the overland price doesn't feel right, especially if it exceeds 150. Even that might be a bit too high.

Very Rares
Regeneration - health/turn is worth as much as a cheaper common buff (is granted by a global, like holy weapon or chaos channels). Reviving the unit is worth the unit's cost times the number it revives. This is hard to even guess but as the most expensive units had their cost cut in half, it would make sense to make this cheaper. Considering 180.

[Nelphine]

Might also consider changing the ratio of combat to overland to 1:4 instead of 1:5? Haven't put a ton of thought into that but since the most common spell type to be usddd in both is buffs?

[Seravy]

I think only buffs are affected by that ratio currently, yes.
However, I don't think that's a good change.
Assuming we decide on a specific overland cost for spells, the change of ratio means the combat cost increases by 25% (for the same overland cost). Buffs are already a weak choice in combat because you can affect one unit per turn - that's a fairly minor effect compared to what you can do otherwise (summon an entire new unit, kill an enemy unit, kill multiple enemy units, globally increase/reduce stats of all units on one side etc). I don't think combat buffs need to be more expensive than they are. Like, for example, Heroism. 15/75 is about right. With a ratio of 4, that would be 19/76. Similarly, Flame Blade 10/50. That would be 12/48 after change. Which makes it more expensive than a usually much superior Fire Bolt. Likewise Holy Armor and Bless would be 9/36 instead of 7/35.

No, I think the ratio of 5 is fine. This would make them cheaper than they are if we adjust overland costs but that's acceptable. These would be the combat costs assuming we agree on changing overland as listed above :
Regeneration - 36. Seems fine, it's basically a weaker "raise dead" - only works after combat.
Lionheart - I think this has to be special-cased as in-combat healing is more valuable than just a buff.
Invulnerability - Would put it at 30. This is fairly good but not affecting game balance. If the spell is relevant in stopping damage, you only need it on one unit to win in most cases. If it's not relevant (attacker has single figure, strong units, or resistance effects) then it doesn't matter how many you use.
Invisibility - would drop to 30. Fine as is I guess - your other units will still get killed by ranged attacks. Also you can make all your units invisible for 80...
Magic Immunity - This would be 35. Kinda cheap for what it does, but same as above, the enemy can still kill all the other units you have. Also, if they can't kill a magic immune unit then it only takes one of them to win the battle, like invulnerability. In fact, a lower cost means it's easier to dispel, so it might even be weaker, as it's easier to remove once the wizard is out of skill for the combat.
Iron Skin - Would become 24. A pretty low cost for such a significant benefit, especially as it's easier to make sure the buffed unit is fighting (armor works vs everything not just ranged where the enemy is free to pick targets regardless of positioning), but ultimately this is weaker than invulnerability. If that's fair, this is too.

[zitro1987]

Lionheart Idea 1 - Make it overland-only, drop cost to 150 or increase max hearts to 12 ... a bit repetitive in combat as it's I believe the 5th healing spell (healing, heroism heals, lionheart heals, mass healing, and the 'rare' healing spell)

Lionheart Idea 2 - combat/overland, reduce max hearts to 6, but increase attack/resistance bonus to 4 or 5.

[GermanJoey]

from the ones you listed, these two stick out to me:

1.) Elemental Armor - comparing this spell to True Sight isn't a fair comparison, IMHO. While its true that they both provide immunity to one type of attack, the difference is that True Sight protects against an uncommon damage type dealt by only a few creatures/spells in the whole game, while Elemental Armor protects against one of the most common and effective damage types. IMHO 75 mana is criminally underpriced.

2.) Lionheart - just looking at overland casting, IMHO this spell is a lot more useful as a defensive buff than Invulnerability. Lionheart on your heroes and on particular Armorer's Guild units to cover various instant-death scenarios. First of all, that +3 resist is a big deal as it will push this sort of high-end unit up to the point of resist-immunity from various nasty curses, like Confusion, Shatter, Black Sleep, Possession, Massacre, etc, even under effect of Black Prayer. Second, Lionheart is much, much harder to dispel than Bless, so it's a useful fail-safe even if Bless would be enough to cover the unit. (and Bless + Lionheart will be enough to protect it against Annihilate). Then there's the hp. That +8 health generally will increase the unit's hp pool by somewhere between +50% and +100% depending on the unit, and remember that Charm of Life is considered one of the best Very Rares while only giving +25% hp. (and that stacks with Lionheart) Finally, the +attack is extra effective because you'll also have Holy Weapon (or Holy Arms)

[Seravy]

While I agree that Elemental Armor is very good, it's intentional - Magical ranged attacks are the best attack type with no real drawback except the spells that work against it. So those spells need to be above average to keep attack types balanced.
(and btw if we compare to Guardian Wind it suddenly doesn't look that good - GW provides complete immunity for less mana - the difference is magic ranged attacks are better than bow attacks, so they are more widely used.)

A minor raise like to 18/90 might be a good idea anyway.