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
I donate 100€ to KDE every year. I consider it my “windows license”, since it was the DE that allowed me to escape from windows 7 years ago.