Maybe donate 50 cents for every hour you used the software and it was useful to you.
That would be 1000 €$ per year if you work with Linux full time.
Let’s see some commercial software:
Microsoft Office 365 is 70 $€ per year. Adobe Suite around 700 $€ per year. IntelliJ IDEA about 170 $€ per year. Affinity Suite is 170 $€ once. Reaper is 60 €$ for a discounted license. Full featured media player like Elmedia costs 20 $€. BBEdit costs 60.
The FOSS windows and Mac FTP client Cyberduck asks for a minimum 10 €$ donation. It won’t prompt you for a donation if you bought a license. The Duck applications are all pretty nice.