I think things have changed since the Nixon/Ford days:
Presidential Food Service provides worldwide food service, security, and personal support to the President and First Family. Additionally, they provide gourmet meals and support catered functions and Social Aide Dinners for visiting heads of state.
Presidential Food Service operates the White House Mess Executive dining rooms, provides a carryout service, and provides complete catering coordination to the White House Complex.
They also provide logistics coordination for White House Mess personnel and valet services for Presidential trips and events.
In 2001 the salary increased to $400,000 plus $50,000 for expenses. Because the president’s official duties incur far more expenses than the expense budget can cover, agencies of the federal government often assume responsibility for presidential events.
The Presidential Food Service is part of the White House Military Office (WHMO).
I REPEAT...The president pays for his and family’s food. Obviously not when it’s tied in with other functions...
http://answers.google.com/answers/threadview/id/726988.html
From the bottom...”Both Nancy Reagan and Barbara Bush noted in thier memoirs that they received a monthly bill for the food purchased for the private quarters”