Not necessarily. If I sell 20 books at $1 in one month, I've made $20. If I put that same book online for "free" but with ads on the page and I make $30 in one month, I've made more with the "free" version of my book than I did with the paid version. I may not have made the same per reader, but that's something that each author will have to deal with on their own terms.
And a lot of those web-readers may buy the paid version so that they can have a nicely-formatted off-line version for their eReader.
None of that presupposes ads IN the book.
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