I worked for a place that had a set rate for accomodation and a flat rate per diem based on location. E.g. if the rate was set at $200 for accomodation, you could stay at a holiday inn for $100 and pocket the difference or the Four Seasons for the full $200 (FYI - I stayed at several at an even lower corporate rate). Same for per diem - flat rate and do as you wish and pocket the difference, if any. If accomodation and/or (some) meals where provided that component would be deducted. Very simple, cheap to administer, and based on research cost neutral to the bottom line.