[Daynarr]

The costs aren't different.

I was just referring to max damage you can do with them early, not the cheapest way (6 vs. 2 ogres is quite a difference). Of course, getting 5th level in nature also enables you to cast Mother Oak and Gift of Health so its sort of "side effect" and getting Mother Oak early would make up for nature games you spend before.
Panganea can do it too, probably even faster, but they would have trouble with water nations and won't have Hydras to accompany those Vine Ogres (deadly combo).

On smaller or special-site-rare maps its probably not possible but on larger you will probably find enough nature sites to get this done (of course there is a luck factor in it all).