I'm not sure that any valid precedent exists for ebooks. When you look at advertising in print media, it is primarily seen in periodicals. Distribution is timely, circulation is known, and there is a lot of research into audience demographics. Print books are almost the polar opposite. So chances are that you don't see advertising in pbooks because advertisers aren't willing to pay. The opposition of readers is probably not a factor at all.

Ebooks are different though. Advertising can be inserted at the time of sale, or even at the time of reading. Advertisers can pay according to the actual circulation, number of views, or even click-throughs. And hey, you can link together a readers' purchase history to target your advertising even better.

That being said, I don't know if it's enough to bring down the price of books. Even if the readers of ebooks are worth more, the average reader would have to spend several times more on the advertised product than they are saving on the book.