[GeoffR]

Almost all of the ebooks I buy from the big publishers have far more errors than the paper versions, including many spelling errors that would have been caught by even the most basic of spell checkers.

I wonder how much money those publishers are losing because of this? Would you pay extra, and if so how much, for an ebook that had some form of error-free guarantee?

The guarantee might not be absolute, perhaps just that there is no more than one error per 10,000 words. I would gladly pay an extra 20% for a book that came with such a guarantee.