From Jodi Kantor’s The Obamas, pages 90–91:
Even the president made uncomfortable jokes about why his wife needed so many things. Behind the scenes, aides said, the Obamas were concerned about money: the president’s books could only sell so many copies, and it would be years until he could write more and the first lady could write her own. From vacation rental homes big enough to accommodate the Secret Service to all the personal entertaining they did at the White House, their lifestyle had grown fearsomely expensive.
Lest someone claim the book is an anti-Obama smear job, Kantor began covering the Obamas for the New York Times in 2007 and based the book on multiple interviews with 33 different White House staffers.
Some further perspective:
President and Michelle Obama’s combined adjusted gross income in 2009: $5,505,409.
President and Michelle Obama’s combined income in 2010: $1,728,096.
President and Michelle Obama’s combined income in 2011: $789,674.
The Obamas’ combined adjusted gross income for 2008, their last year before moving into the White House, was $2,656,902.