I have to say registration fee because those two keywords do not correlate (so aren't searched that often)
Just because it's got the word "porn" in it doesn't make it worth money. The 2 words have nothing to do with other & doesn't make sense. Honestly, you'd be lucky if you got reg fee
You can mix basically any work.. that doesn't relate to porn and then porn beside it... 2 keywords need to mix together well (for search popularity)