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.
I would say, this can easily be sold at 5 USD/note.The other notes are much shorter in circulation:
$1: 1.8 years
$5: 1.3 years
$10: 1.5 years
$20: 2 years
$50: 4.6 years
On average an ad on a bill might only be sold for 1 USD, however, given the fact that each day the bureau of engraving and printing produces 38 Million USD bills, this could result in 14 billion USD, more then the actual printing costs.
If the design is well choosen, it would not change much of the bill´s appearance as we know it, look at some examples: