This is a really hard part, deciding how to make money with your app/game...
You could try what they did in the olden days... Make 1 game free, then have more games that cost...
(I know games like commander keen did this) or even a demo version... like Jazz jackrabbit 2 (Love the classics)
In game items for real cash seems to work well, but is really hard to balance/make it fun for everyone (for the high paying players as well as the no paying players...)
I do like the approach by some games, where you can watch an ad for in game things... so it doesn't force you to watch it, but you choose to...