Hi Guys,
A lot of the fonts I like are requesting a donation if I would like to use it for commercial purposes? But what is an average amount? I don't want to be too cheeky, or too generous? The ones that ask for specific amounts seem to vary from 5 to 40?
Thank you in advance

I think 20€ or 15£ or $30 seems to be a reasonable amount.
But it's just an opinion.