IIRC there's basically one payment processor, whose name I can't recall, that ~100% of NSFW sites use. And their fees are very high, given the risk of processing those payments.
CCBill is the primary one and they charge nearly 4% per transaction and routinely hold transactions for up to 6 months sometimes. They also have high registration fees but they are one of the best for high risk industries.
They have a 3.9% ("green") tier without a high-risk fee, but I don't think the adult companies qualify that one.
I could be wrong, but I didn't find a clear breakdown of who qualifies for each plan on their website. I'm guessing the 3.9% plan and their non-profit plan are there so they can present as a normal payment processor.
