I've just read the review, Anth, and I note from it that this WG pays for the cost of the hotel & the meal, which bring the punter's net outlay closer to £400.
The last 2 Overnighters I've had have been with a WG who charged me £350 but expected me to pay for the hotel room and the meal in a nearby restaurant before we got down to doing the deed, including DFK.
If I do any more Overnighters [the WG I saw has withdrawn them from her Profile - nothing to do with me, I hope!], I'd be looking at a net cost around £400 - £450. I wouldn't think of going above that.