Whether an ON is actually worth paying £500 or whatever the amount, is obviously down to the individual but it's a different scenario to a normal 1,2 or 3 hour punt. It's an event, an experience that can include all types of sex and other things.
For instance I only see two ladies on an ON service, a foreign lady every few months when she comes to London (£500) and a parlour girl for a bargain £300. Adventures sex is interrupted with interesting chat and flirting, sometimes a meal, drink or both. A relaxed no rush atmosphere creates a different experience.
I like both ON and conventional punts but it's just my opinion for what it's worth but good luck to the OP.