This artist has estimated the ROI of the media buy that would involve printing custom US dollar bills and created some spec work: "The average 100 USD Note is circulating for 7.4 years, it changes hands on average 3x per week, so each ad on a 100 USD note is seen by more then 1000 persons." The U.S. prints 38 million bills a day. Sell the space for a buck each -- for a Franklin, that will be $1 CPM over each bill's lifetime -- and add $14 billion a year to the ailing economy.
Hey, if it works for the U.S. Postal Service that allows branded stamps, why not for the Bureau of Engraving and Printing?