In US dollars, since that's the currency I spend.
For regular hardcover books -- not book club editions -- I'd pay under $24 without thinking twice. (Love the warehouse club book prices!) For the larger paperback books and I'd pay $10 - $15, depending on how they feel in my hands.
For mass market, I wouldn't pay anything. I prefer the larger formats.
That said, for ebooks, I guess it would have to be less than $20 just released, under $15 when it comes to paperback, and then under $10 when the mass market version is released.
I don't mind the idea that if you can't wait, you pay more, and I have done it myself when I want a book badly enough, but I don't think they should be the same price. I like the idea of 40% off the price of the comparable paper version, to make them slightly cheaper than their counterparts, but not dramatically so.