Congressional Research Service. May 17, 2012.
For security and other reasons, the President, Vice President, and First Lady use military aircraft when they travel. The White House generally categorizes the trips as fulfilling either official or political functions. Often, a trip involves both official and political, or unofficial, activities. When a trip is for an official function, the government pays all costs, including per diem (food and lodging), car rentals, and other incidental expenses. When a trip is for political or unofficial purposes, those involved must pay for their own food and lodging and other related expenses, and they must also reimburse the government with the equivalent of the airfare that they would have paid had they used a commercial airline. When a trip involves both official and political activities, a formula determines the amount to be reimbursed for that part of the trip involving political activities. Whether a trip is for official or political purposes, the Air Force pays all operational and other costs incurred by the use of the aircraft. While the travel policies of specific Administrations concerning the reimbursement of expenses for unofficial travel generally are not publicly available, it appears that policy guidelines developed by the Reagan White House have served as a basis for the travel policies of subsequent Administrations.
http://www.fas.org/sgp/crs/misc/RS21835.pdf [PDF format, 8 pages].