I would consider £600 for three hours with certain SPs 'cheap'. I wouldn't use that word in front of her hehe

Of course the difference with the parlour experience and indy. Dozens of times I've seen SPs in a parlour setting that could easily earn treble what they did in a parlour. But (at least the ones I spoke to anyway) love the convenience of not having to deal with admin/security/accommodation/messages/calls/cranks/cleaning/burner phone et al.
Service users will have their own perceived limits. These are related to opportunity cost (not to go too deep!). This hobby is my main fun, so what I would be willing to spend (a lot) is not the same as others. Then you have people limited to what they can literally afford.
I think maybe the development in the industry over recent years, perhaps since lockdown somewhat enforced it is that it's a lot easier to be indy than it used to be. More reliable, high quality indy providers impacts the market, including the 'labour' market. So there needs to be a better earning potential to attract the stronger SPs to work in parlours. That would be my take on it, from a user perspective.