Here's a few popunder ad networks. I don't use popups or popunders on my site, but do use the first 2 ad networks listed for their cpm banner ads. They also have popunders where you can put a 24 hour cap on them if you want to so that they don't annoy your visitors so much. Their popunders pay around $3 - $6 cpm I think. I'm sure it was around that way when I used their popunders last.