I've got mine set at $16 at the moment.
Best thing to do is look at how much you're making per day, then increase the price - keeping an eye on the profit that day.
At some point you'll notice it starts to go down instead of up, so set it at the highest.
I do drop it (if I need to) for the achievements that require a certain number of visitors (and then put it striaght back up again)
They seem to have stopped the being able to help animals when you adopt one, you just have to wait for one to pop up while you're playing (I find always have Zoo World open in one tab helps them pop up a bit more frequently).