I’ve got some standing orders of 1 or 2 euros every month for bigger projects like grapheneos and kde. And for small apps that I actively use, I try to donate 5 or 10 euros now and then. It’s not much but I believe if everyone is doing their part we succeed in the long run
Some years I donate nothing. Other years it’s about 50 euro… Depend on a lot of things. I believe in giving to the free software, because that makes it better, and since I’m a user, that’s great - and I like to keep it running.