It’s a good question to ask and I hear it often, the answer is:
This changes and varies depending on your traffic demographics and the type (is it an iPhone user, a PC desktop or a MAC for example).
The only real way to find out how to get he most for your adult pop-under traffic, is to test different brokers.
TIP: When you test, make sure you use the same traffic source and test with a good volume. Don’t decide when you’ve sold 1000 visits. Decide when you’ve sold 10000 or 100000.
Test each for a week or a month and then decided who you want to sell your adult pop-under inventory to 🙂