M&O Rewards - Apps on Google Play
If you don't want to use real money, I use Google Rewards for easy pocket change towards in-game purchases. I've been using it for a couple of years. It tracks your gps
Google Play LiveOps - beginners guide for app marketers
mRewards - Games & Earn Money - Apps on Google Play
Miles - Travel, Shop, Get Cash - Apps on Google Play
Google Play revenue cut lowered to 15% on first $1m
Rewards Now by CAL's Convenience Stores
mRewards - Games & Earn Money - Apps on Google Play
myWorld Partner - Apps on Google Play
Miles - Travel, Shop, Get Cash - Apps on Google Play