Her services may be specialist but they don't incur any expense on her part.
Since when has the cost of providing the service been the determining factor in WG prices?!
Supply and demand, that's all there is to WG pricing. They have to select a level of the market in which they wish to compete, depending on their assets and willingness; they can go Aldi, Tesco, Sainsburys, Waitrose or specialist deli. if they look skanky there's still a market for them. If they look like princesses that's a different market. And if they are willing to be tied by a stranger to a St Andrew's Cross frame whilst being fucked then tied to a bed, degraded and spunked on well then that's another market.
If a girl charges £500 an hour she won't get my business, but she's clearly not looking for it at that price. If a girl charges £50 more than a girl with identical services in the same vicinity she'll lose many bookings purely on price. But some men will sometimes pay that extra £50 or more for a particular different kink and I did with Charlotte just to try a sub session in a dungeon playroom and tick it off my bucket list.