I mentioned that I did not take into account the values of various abilities
I also mentioned that the values of individual attributes are not equivalent.
Your monster does not exist in practice. The values of attributes do not have a linear increase in value per point. They have a value (negative or positive) based on their statistical deviation from the mean values of their opponents'. This makes it difficult/impossible to determine a "cost" for a given point. If you take a look at the creatures I compared, they all fit into a relatively small variance with some outliers: The Wyvern has roughly 20 hp fewer than the others. The Cave Drake has 22 protection. The Vine Ogre has 50 morale. The Wyvern has the best combat skills with 13 Attack and 11 Defense--which since it is at or slightly above the values that the majority of national troops spawn with makes it much more valuable than the skills of all except the vine ogre who has 12 attack skill--allowing it to successfully attack units. It can't fly, but it doesn't eat either. Is Flying better than Doesn't-Eat? Probably, but it restrictys amassing hordes of Wyverns (like you'll see of Vine Ogres). This, combined with the morale of a wyvern (the lowest in the group) makes it less effective as the game progresses.
Would
you buy a 3 gem Wyvern?
I'd save mine up for Draconians (which aren't quite as good, but the spell grows cheaper as your Draconian Chief summons more draconians) or Air Queens. You get one chief and 16 draconians for 45 gems--if you don't count the chief's value, since he's basically just a draconian producer, each Draconian cost 2.81 gems when you initially cast the spell. After 10 rounds of summoning, your draconians cost an average of 1.73 gems apiece. It goes down from there. Of course Draconians is a 6th level spell, but saving up for it is better than buying 3 gem wyverns
2 gems might be ok--I'm not sure. I haven't done any actual testing.
If you don't like my evaluation, come up with your own rather than simply attacking mine with arguments I already foresaw