In my case, for donations on a forum... nothing converted better than £3 per month.
Why? Because it's been translated into "a beer a month", as someone initially said that surely people derived value such that if they met me once a month they'd get a beer in for me... and lo, that's exactly what people started doing online. Prior to this comment I had real trouble pulling in any donations, now people buy the servers and I a beer each month (via £3 donations).
My thought on this a while ago: People have to be able to visualise it in a human way to them.
They can visualise a pound coin or dollar bill and say, "That's not much", and they can visualise a pint and think "It's the courteous thing to do". They can't visualise 67p very well at all.
