How do Google Play app updates work? Are they free? Let's say I paid for an app, and after some time there is an update for this app.


Is this update free?  Will all future updates be free?
Or do I have to pay some fixed % of the current price of the app?
If it can be either, is it up to the developer?
According to the Google Play Business and Program Policies, you may re-install any app you have purchased, unlimited. (also if the app has been updated since you bought it).

Based on my experience it works like that indeed: I have updated and re-installed paid apps without any issue. Please note that the re-install/update also works on any other android device that uses the same Google account.