Depend if you linked your credit card with the game.
If not, it's only $5,99/upgrade/stall (not very expensive if you do not put too much stores in your park) except the last upgrade at level10, which is more expensive ($ 9.49) but it's just because this allows changing the price of the sold nouriture, which is cool.
But here's a little trick, if you pay a monthly subscription ($18,99 but the first month is free), after that it's only $2,99/upgrade/stall.
You can also change the color of the wall for free if you a subscriber. Otherwise, it's $0,99 (Exceptions made for "legendary premium paint pack" which are necessarily more expensive, but these are only lengendaires paintings, so it's normal). Oh, and more importantly, everything I have said is false.