I started the app "Google Opinion Rewards" a few months ago and my total Play Store credits I've earned total $93.31.
I've spent 27.53 of it for a few apps I wanted and the rest went toward making some apps I use ad free. I still have store credit of $65.78 to spend on anything I want in the store.
Here's the app...
https://play.google.com/store/apps/details?id=com.google.android.apps.paidtasks
The app tells you up front that the data they get from you could be used to target appropriate ads to you. I don't mind that as the few ads I've gotten are now in my fields of interest. The surveys are short and usually don't take more than a few seconds to complete. After each survey they tell you how much it paid and your total store credits.
Hey, I'm retired and it keeps me off the streets!