Anonymous:
$5 - Most people.
$50 - People that seem honest, have some short history of being honest.
$500 - People that have a well established history of being honest. (Last stop for anonymous transactions.)
Edited out the bits I didn't want, but the sentiment is spot on. For anonymous transactions, how do you feel about it if it was a gift or donation where you don't get the money back.
If I give $5 to someone on the street I wouldn't expect it back - especially if they need it for food or a place to sleep.
$50 is more donation territory for a lot of people, but getting it back eventually or some other way is nice.
$500 or around 100 bitcoin - never hurts to ask and there is some small beginnings of a trust network/grading system evolving.
It also depends on how much you can afford to lose. As I've gotten older, the amount increases.