Let's put it in that way:
100 crystals will get me around 100 obsidian in Excelsior (yep, slots are not equally distributed). Also, I can get some crystals (around 20%) that can be "reinvested". I would need moreless 400 crystals to get 495 obsidian, that is 400 crystals x 29 gems/crystal = 11.600 gems for the buff... this is hilarious, isn't it?
I just prefer to believe that the prices in merchant are set by some algorithm that either employs a certain degree of randomness (or sadism) or try to exploit drunk players...