It's been three months since I posted my novel online for the world to read for free, with a tip jar as compensation medium. I think we have enough data now to tell how well it might work for other artists.
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As I write this I have received about $760 in tips from 85 people. Thanks guys!
About half of the tip money came from the initial introduction, indicating either that K5'ers are extraordinarily generous or that I was getting extra bonus points for my other work here. K5 also generated my largest tips, three of $20, a $30, and one of $45. (Did I mention that some people seem to like the book a lot?)
The easy answer to this question is 'whatever someone is willing to pay for it' -- and nowhere is this more accurate than when using a tip jar or donation-based payment structure for content.
There's a great article this week in the ClickZ newsletter about just this subject. I've linked to the 'print-page' version below, as their links tend to stay active the longest.