Ah, actually, Project Wonderful doesn't allow bids of 0. 1 cent is the lowest you can go, which I honestly think is a good thing, so that people are forced to actually pay for advertising (from an ad host's point of view). You still don't pay if there's no competition for the ad, but if there is, and they bid 1 cent also, you then start paying 1 cent per day. And based on my recent advertising experience, it's not worth it. I try to stick to advertising with an effective cost per click (unique) of less than 5 cents. I also try to avoid advertising that amounts to less than 6 unique clicks per week. If you don't have much of an international readership, then you might as well just come up with some decent default images to display.