He is changing the cost to 30mU in future versions. Plus I only assumed as I'm not sure how to the costs work out so until you see otherwise you can assume my math is wrong.How do you reach that 121 tree number? Using 15 apatite gems per tree?
Arboretum costs 40mU. If that is per tree, then one fertilizer can harvest/replant 2000/40 = 50 trees. So 28.5 stacks are 28.5*64*8*50 = 729600 trees.
edit: And yes looks like I screwed up the math badly =/