This is probably an age old question, but I just couldn't find the answer to my question anywhere. I have 4 earth habitats. Two are level 2, one level 4 and the last is level 5. Now what I don't understand is why does it cost 400 tokens to upgrade from 2->3, 450 tokens to upgrade the level 5->6 and craziest of all to upgrade the level 4->5 requires 500 tokens!!! I just don't get it.