In open source development it is a lot more than 0€

It's not a very grateful job. Many developers provide an option to make donations, but often don't promote it too hard. So if you do not have a dedicated person organizing a crowdfunding campaign but concentrate on development instead, it's nearly impossible to reach such sums. Often you also get pressured by the community.

Issue 752: WHY DOES FEATURE XYZ NOT WORK??? THIS IS BULLSHIT THE DEVELOPERS DONT CARE