In my experience (primarily hearsay), guys who are "genuine" are also a bit stingy. They figure they're taking you on a holiday and that you should be happy with flight, room, board, their company and maybe a tiny bit of pocket money. Even telling them that you'll want to make what you'd have made at home (plus flight, room and board) turns a lot of them off.
Beware guys who offer a lot of money up front.