According to industry sources, ATI's HD 5700 series might sell much better than the HD 5800 as the key issue for HD 5800 is still availability. Both series are selling quite good, but partners are still complaining about the HD 5800 series, as AMD simply can't produce as much cards as partners need. The HD 5700 on the other hand is quite good, it delivers quite a "bang-for-buck" and also brings all the features, like DirectX 11 and Eyefinity, that you can find on the HD 5800 series. Availability for the HD 5700 series is decent and partners had their cards delivered to retail/e-tail shops days before the official announcement.

http://www.fudzilla.com/content/view/16005/1/

You'd expect mid-range to sell better because of the price, but in this article they say it's because of the availability.

I wonder ... if high-end would be available everywhere, would it sell much better? Or just a tad better.

Smaller market share because of higher price, to much availability would only harm the manufacturers because they can not get rid of their products. Lowering the price then is just plain stupid, you don't sell a Audi A6 for the price of a A4, unless off course new gen is there.