I'm a fan of prices for out of print titles that compare to used paperbacks ($2-4) and will buy in bulk at that price point, but up to $7 is reasonable. Maybe there are people who will pay $13 and up for backlist ebooks, I'm not one of them, but I don't buy many $13 new ebooks either.
I think we'll see publishers experiment with prices for these, trying to maximize volume and profits. They will probably be reluctant to go very low for authors still writing, so as not to erode the value of their upcoming books, so there might be different limits for dead versus living authors. I see backlist ebooks as a huge (mostly untapped) market that will explode.
ETA: The Lawrence Block backlist ebooks are a good example, HarperCollins had initially priced them the same as the paperbacks and this week they reduced the prices to about half that ($3.99). Block writes about the pricing strategy in his
blog.