You can make a little bit of extra pocket cash without too much effort, or a lot of money with a lot of effort and capital. It's like any other business - you have to have a good product, a good price, and promote it. And figure out what your business model is. Are you making money from subscribers, advertisers, or a bit of both? I tend to think, as with any syndicated content, that most money is made from advertisers.
When you say you've seen sites for independent providers, understand that they are generally making money from being service providers, not content providers. Their sites are promotional rather than the product itself. A few have "members only" side businesses and might make a little, on maybe a notable sum if they are constantly providing fresh content and reaching new subscribers. Most seem to entice subscribers with the idea that they might "meet" lucky members.
Internet Porn is a machine. There's little magic to it. Content, price, promotion. Then you sell the successes of that to advertisers. The well-established principles of business still apply. If you treat it like a hobby, it will pay off like a hobby.
Web development isn't cheap - the bigger sites have sizable infrastructure supported by large capital investment and high ongoing maintenance costs.
If you wanted to start with a small respectable site where you could manage content, memberships, with a payment mechanism, I'd budget minimum $5000-$6000 for design and build. This is a starting price. Then you have to look at advertising programs. These can be a clickthrough cost or fixed scope media campaign. These are ongoing costs because subscribers fall away faster than you can generate new content.
If you just wanted a hobby site, were willing to learn some basic HTML, and used Paypal or other payment provider, then you could get off cheaper, but it's more ongoing labour and people are less likely to fork over money for something perceived as less valuable.
I'm not trying to discourage you. It's just not a small project if you want to make a goodly amount of money from it. You could squeeze out a few dollars without a huge effort, though.
Feel free to PM me again if you have more questions. We could chat on the phone if you want.