I dunno because I've never used one. I enjoy doing it myself. I'm told there's a processor nearby who'll do it for $75, which includes burgers, sausage, etc. That's sounds like a great deal to me, but part of the reason I got into hunting was to not have to pay for meat (got a ways to go to cover the cost of the guns and gear though, but it will happen).
As for whether or not they take payment up front or in arrears, I'm not sure. I doubt any processor would take game not field dressed, but like I said, I've never used one. Are there people that hunt that won't field dress? Seems those people shouldn't be hunting, IMO.