The Firefox add-ons blog just posted some results from their "Contributions Pilot" where they were testing accepting donations for add-ons. http://blog.mozilla.com/addons/2009/10/19/contributions-a-lo... Interestingly, the $5-$10 range seems to be the sweet spot there as well. In this case though the add-on author suggests an amount to donate which might skew the results compared to Goo's results.