It´s like making an in game -shop, where you can buy stuff (upgrade/levels/characters/items...what ever you want) for real money.
You can just make an own money for you game....like gold and sell it for
0,99$ = pack of 99 gold
4,99$ = chest of 1000 gold
etc etc.
That´s how most of the games on Appstore handle it.
As I see you are a new user to C2...don´t even bother with this. People won´t buy your stuff, unless your game is really great. Concentrate on making a working super addicting game first!