I will say this. If you don't know don't do it. You need to know the answers to a few more questions than this and the ones you asked are basics.
As ASM said it can cost very little or a lot of money. Don't even dream of competing with Bangbros, they are out of your league. They are out of my league as well and I know the answers to your questions.
But as I set up a site a few years ago here's the costings.
Hosting, around $2,000 for a start up. When you get more traffic then you need to spend more and keep spending as traffic builds. BW might go on top of that.
Scripts, $2,000 for CMS unless you have a LOT of content. Start a site with CCBILL and use there affiliate script. If you grow then think about NATS and decide if it's worth it.
Design, $1,000. Forget about all the fancy designs, unless you have poor porn. Show surfers the porn, throw open the site so surfers see what they are buying. You don't need a fancy design, we are pornographers not Interior Decorators.
Processing, might be free but ASM gave you the figure.
So without content $10,000 is a reasonable figure to cover it.
Then submit your site to a few review sites, see what they say and make changes. Then submit to all the others, assuming they did not pan your site.
Within a few months you will have recouped your $10,000.
EVERYTHING rests upon the quality of your porn, if you have good porn you then need to just tell people you are there. Good affiliates are not fools they know the difference between good and bad porn.
If you have bad porn you are fucked, because you obviously do not know enough to work the traffic game, getting thousands of affiliates sending you traffic that converts badly.