There are several things to think about:
1. Content - you can't just cut and paste from the news groups. Well, I suppose you could, but why would anyone pay to get porn that they can get for free? You also have to update the content on a regular schedule, say once a week.
2. Bandwidth - you will probably have to host your own server, since most providers frown on hosting a porn site using their lines. There are some 3rd party ISP's that let you store your content on their servers, but that can get expensive since they have the market cornered.
You will also have to pay for the bandwidth you use. If you get a lot of traffic, then you may be cut off (not good), or you will pay thru the nose. You are better off getting a dedicated high-speed line and your own server, it is usually cheaper in the long term.
3. Liability - you need to setup some sort of credit verification scheme to keep the kids out. Unless it is a proprietary credit billing, you might not see much, if any, of the $29.99 or whatever you charge.
4. CONTENT!!! (again) Streaming movies, interactive features, live chats, you might need some or all of them. People want them, and that is where the money is. I can get pictures and many movies off the newsgroups for free, so why would I pay for them?
A good friend setup a porn site with the same intention that you have, get some picture up and watch the money roll in. Never happened. Cost him a bundle.
Av.