Office of Management and Budget: Circular A-21
ORI Introduction to RCR: Chapter 8. Collaborative Research
a. General. Travel costs are the expenses for transportation, lodging, subsistence, and related items incurred by employees who are in travel status on official business of the institution. Such costs may be charged on an actual basis, on a per diem or mileage basis in lieu of actual costs incurred, or on a combination of the two, provided the method used is applied to an entire trip and not to selected days of the trip, results in reasonable charges, and is in accordance with the institution’s travel policy and practices consistently applied to all institutional travel activities.
b. Lodging and subsistence. Costs incurred by employees and officers for travel, including costs of lodging, other subsistence, and incidental expenses, shall be considered reasonable and allowable only to the extent such costs do not exceed charges normally allowed by the institution in its regular operations as a result of an institutional policy and the amounts claimed under sponsored agreements represent reasonable and allocable costs.
c. Commercial air travel. Airfare costs in excess of the lowest available commercial discount airfare... (Link)