I shoot porn for a living and you might be surprised at how it actually works. The vast majority of pornstars are contract girls. Meaning they are not allowed to work with anyone outside of their company without permission. The girls that advertise as pornstars are usually girls that left the mainstream industry and are trying to piece together odd jobs for assorted web sites.
When you find a girl advertising for work as a porn star, she will almost always fall into one of three categories:
Retired. Got too old for mainstream porn, regardless of what she looks like. Can't get a contract simply because she's over 35, even if she still looks 21.
New. Just got into the industry and pretty much lacks any experience of note. Hiring them isn't much different than hiring an escort.
Blacklisted. If a girl fails too many STI checks, she will be blacklisted from becoming a contract girl. She might be clean now, but she has a history of not being safe.
As far as rates, most girls don't post rates. If they do, they are most likely just an escort claiming to be a pornstar. When I hire girls, I give them job details and an offer of pay. They can try to haggle for more money, but I'm not coming up by much. They either want the job or they don't. If they pass, there are ALWAYS a ton more girls willing to take her place.
Note: I don't think I've ever paid more than $2500 for a shoot and that was a two day shoot. Porn actresses are almost always cheaper than escorts of equal caliber, but that's because escorts work so much harder.