There are no limits to your earnings. You make money through your games in two ways – banner ad impressions, and when the gamer buys coins using In App Purchases (IAP).
Revenue from banner ads you receive from selected AdNetwork.
Revenue from IAP you receive directly from Google.
You can make a good game in 30 minutes, upload it to Google Play, and if it becomes popular you'll have a regular passive income for years to come! We have seen this happen and it can happen to you too!