BTW- I did a similar project (non porn) with a little more functionality than adultcheck and charged $8k. However I didn't have to port into a CC system. It did have a secure user login system though.
I have also worked on not so ethical (porn) projects and I have charged more for them just because it is such a large market and it is harder to get people to do the work.
I would charge somewhere between $6k and $12k, depending on the effort. This, to me, is more than resonable. Like the post before, figure out how much you want to charge an hour and how long it will take and that is your number. I charge about $40 an hour for a standard project. I would say about 200-250 hours to complete this project. So, $8000 - $10,000 total. Then again it is a "porn" site so I might charge $45 an hour, $9000 - $11250.
Does anyone else agree?