The limit to the amount of upgrades you can make on a creature is a very good idea.
The thing is, the costs compared to the earnings are crazy!
100 boosts per tier when you can only get 4 daily makes no sense. That means close to a month just for 1 tier.
I suggest the ratio between the two be lowered. Upgrading 1 creature per month is too slow (or get rid of boosts altogether. Either way makes sense to me.).