Yep. There's a big difference between doing a group booking where
* you're the only one there (odd how the anonymous other one isn't going to be there, because this is what "may not be there" translates as!) and
* they control the location, and
* the men all know each other, and
* it's packed into an hour,
* with eight of them wanting constant sexual contact, including stuff like bondage and who knows what else (filming?)
as opposed to a more usual 'party' where
* there are other workers, plus almost always someone else there you can more or less trust, and
* it's at a location of your choice / run by a continuing party / 'brothel on wheels' setup, and
* the men have booked separately, and
* it's spread over two or three hours, including the opportunity to chat
* with, usually, a lower client:worker ratio.
It's also different from a recreational gang bang, for example as part of swinging, largely because of the cold start and the way that, regardless of what they think happens, you pick the men.
I don't know what you charge, but eight men for an hour is a lot harder work than eight men spread over eight separate bookings. If you want to do it, factor in the cost of someone else there - if they're offering you ?600, they should be paying the same to Ms Imaginary = their real budget is twice that - and of not being able to work for a day or so afterwards.