I thought it was 25% but I think I heard that somewhere and have never actually checked myself.
I still prefer it to advertise then use as a publisher.
Another thing I've found is don't be scared to put lower bids for LONG periods. There will be times when the high bidders reneg their bids and yours will be all that's left
When I still had the box here I saw TONES of people doing that here. It's funny as you can sometimes predict the bid based on the site.