Are you planning to find your own umpires? You could always contact the assigner for the conference(s) of teams you plan to invite. He could give you a better idea of what the rates should be and how to go about getting umpires to work at this event.
Otherwise, you could probably get umpires out there for 50-75$/game plus mileage if you know how to find them and their phone numbers. Three games is enough for one day in my book
Are you going to run three man or two man?