As you're offering your visitors free porn, why would they sign up for regular porn programs?
You need to provide them with something different, a real experience, which is to say dating and cams.
Fling and MeetLocals (dating), which are two sites by Huge Traffic, pay a flat $35 per sign-up.
Ashley Madison (dating) pays up to $200 per membership, especially for US and Canadian traffic, and they support PayPal.
Livebucks, for cams, with 50% revenue share and up to $100 per join.
Of course, if you can niche down these programs with your videos categories, they'll convert even better.