[Verjigorm]

Based purely on the summation of the values of the various units' statistics (Size, HP, Protection, Morale, Magic Resistance, Attack, Defense, Precision, Movement, and AP), the units come up like this:
Wyvern: 129 + Fly - Cold-blooded - Animal
Ice Drake: 140 + Cold-res(100) - Animal
Cave Drake: 153 + Survive (M) - Animal - Cold-blooded
Fire Drake: 137 + Fire-res(100) - Animal - Cold-blooded + Survive (W)
Vine Ogre: 163 + Poison-res(100) + Survive (F) + Need-not-eat + Mindless - 1 x extra paths required - extra conj level (3)

Clearly the vine ogre is worth more as a unit. If you were to change the costs of the above units, one could probably infer that their gem-cost should be 1 considering that you can get more than one vine ogres for a single gem while the 2nd level summons are extremely expensive. However, that fails to take into account the factors listed as +'s and -'s above, differing values of various attributes (1hp is clearly not equal to 1 attack skill in value), and it fails to take into account divergences in the costs of the various low-level summons. Under the current modding ability it is not possible to achieve a true "balance" between these summons. My suggestion, cludgy though it may be, is to reduce the gem-cost of the 4 elemental 2nd level summons to 1 and see what happens.

My prediction is that you'll see groups of them arising in non-nature nations while nature nations retain the vine ogre. Since they can't be hoarded as efficiently, however, they will tend to be phased out later on when better things come along.