I started regularly donating about a year ago. I am now supporting 8 different projects with 20,77€ per month, and it didn't hurt at all.
Why pay for something free? Because free software only exists because other people already paid for it. Someone has to do it. It's time to do your part!
If you can afford a subscription service, then you probably can afford a little donation, too. Start with 0.1% of your income.
Check out @Liberapay or @opencollective