I added up how many gems it would take to buy ONE of all the attractions (192 gems) and FOUR of each animal that I am aware of at level 26 (917 gems x 4 = 3668), plus the two mystery boxes (24x2=48). You also might want to buy an extra breeding and/or cross-breeding nest (149x2 = 298). This equals over 4200 gems.
Then as I understand it, the stuff from the boxes costs gems, and there are some animals and cross-breeds I have not yet unlocked.
In my game, it costs $100 for 580 gems, so you would need to spend $800 on this game, to get "everything" NOT INCLUDING the items from the mystery box. If you skip the boxes and are patient enough to use one of each nest, you only need to spend $700.
O.M.F.G.