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
About 20-30 euros each month. I Always budget 30 and give a bit more to tools I use regurarily and disperse the remainder on projects I tried, liked and felt like I wanted to donate to that month.