Even the payfiles on WMM are quite cheap compared to most of the stuff out there, which is often more around $50.
I think particularly many of the hypnodommes price their files high because they have few, but loyal customers. It's not an easy service to market, so they don't attract many new customers on a day-to-day basis, but the customers they do manage to lure in will often stay loyal and be willing to pay the somewhat high prices. By this I'm also implying that many hypnodommes make their customers more loyal through their files.
I also agree that it's a little hard to justify the purchase when you don't know the exact contents of the file. Just with free files here on WMM, I've experienced many files that were a little off from how I imagined them when reading the description, which, in the case of a payfile, could mean that money was wasted on a file I wouldn't be interested in listening to.