I don't think it makes much sense to categorically set a ceiling on what you're willing to pay.
E.g. I tend not to buy text-only books (paper or electronic) that are more than $10. However I also occasionally purchase monographs, which can range from $10 to $50. Also, if a textbook that could work fine electronically is $40 and the paper version is $80, chances are I'll take the ebook.
On a separate note, in a few years it won't make much sense to peg ebook prices to paper prices. That attitude is pretty much a part of the transitional period.
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