(contributed by Matthew Skala)

Project Wonderful's prices are low, but I don't think most users realize just how low they actually are. I did some comparisons between ads on PW sites and other similar ads sold through conventional channels, and the results are pretty clear: the same ad space sold through PW will command a much lower price. That's awesome for buyers, but it's not so awesome for sellers - which is why I'm listing this under the "Selling" topic. A site that could sell its ads some other way isn't going to want to switch to PW if it means accepting pennies on the dollar of the ad revenue it would otherwise get, and I think that's why PW hasn't become very popular with the big, high-traffic sites yet. Those sites often don't price things in terms of CPD like PW does, but for comparison I'm going to attempt to compute CPD for a range of sites. Let's see what we get.

To keep Project Wonderful moving plenty of cash around, it's important to give advertisers plenty of choice - but to keep supply in proportion to demand.  If you're earning less than fifty cents a day on your site and you've been going for more than a month, you may want to use fewer ad boxes.
Each and every advert on your site is competing with the others on the same website, and if you have too many, then there's no good reason for an advertiser to make that all-important first cash bid.
Remember that most advertisers are looking for the best deal for the least money.  It's unfortunate, but true - nobody's going to just hand you a fistful of dollars for having a cool comic or blog, especially if there are a dozen button ads, a leaderboard and a skyscraper to choose from on the same site.
Competition is always a good thing, but you shouldn't compete with yourself!
It's hard to say what your ideal number of ads should be as it's going to be different for everybody, but as a general rule of thumb, if you ever see "Your Ad Here" images on your site and you're not earning any money, you could probably do with fewer ads.
It's good that advertisers have plenty of choice - but there's a big jump between giving them choice and flooding the market!