I have participated in one, which included five guys and three girls. I think we each paid $400, and we went for two hours. We had a nice large room with two beds. A suite would have been better, but we never felt crowded. Each girl got roughly $700 for two hours work.
I have read orgy threads on a couple of other boards, and the guys always say they want more girls than guys. That's a nice fantasy, but I can assure you it is unnecessary and a waste of money. The average -- or even above average -- guy cannot keep a provider busy for 2 hours straight. Moreover, I think part of the orgy fantasy is multiple guys on one girl, as well as vice versa. I can tell you that the 5 on 3 ratio worked just fine, no guy felt left out at any time, and we all got as much as we could handle.
As far as organizing one goes, the one I did was organized through another board, and most of us had met at formal or informal board social events. Given the limited number of providers who regularly post here, that might be difficult. If I wanted to do one I would pick a provider I knew who was open to the idea, and ask her if she could find a few other girls... maybe their duo partners. I would let them pick their clients who would be interested. Depending on how long you want to go, I would try to agree on a rate where each girl made, say, 150% of her normal hourly rate. In other words, if the orgy lasts two hours each girl would get paid for three hours.
If you want to go with the all-out fantasy of an overnight, untimed orgy, I would try to line up maybe four hobby "couples" -- guys and their ATFs -- who would be open to an overnight with group play. That might help get reasonable pricing, and enough comfort on the part of the girls to participate.