Yeah that's going to vary a lot depending on what the 'assignment' is, where you are, where you're going, etc.It's possible to actually get fairly good deals (from a time/money point of view) if you're just hiring a girl to tag along with you on vacation to some really groovy spot. I've had buddies that have managed this kind of trade for the equivalent of expenses + one or two sessions fees (but they were taking the girls to a nice beach resort).
If you want her to follow you to a conference in Detroit for the week it's going to be a different story . . .