I am currently a consultant, but have also been on the other side of the fence and managed consultants while working for a community bank. Our out of pocket expenses generally vary widely depending upon overnight travel (hotels, meals) and airfare. We generally provide a range of out of pocket expenses if they will be significant (i.e. 10-12% of project cost). Some clients ask for a cap on expenses, and some want everything rolled into one price (accomplishing the same thing).
Switching gears and going back to my banking days, there were times I asked for and got a cap on expenses, and other times I didn't ask. If I thought the actual billed expenses were out of line, I would ask for the detail, otherwise, I wouldn't.
Bottom line, be comfortable with the consultants / auditors that you are using. If they can't provide you with a good estimate of the expenses, you might consider asking for a cap.
_________________________
My opinions are just that...my opinions.