The HD sites are dictated to price wise by the record companies. As such prices are fixed. You might get something at £9.99 at one site and £10.99 at another but that's it. There is a list of sites on another one of the 'HD Download' threads on here.
I would treat all HD downloads with caution. What the poster meant by 'not all are HD quality' was not that the site does 16/44, but that 'hires' tracks are often upsampled 16/44. In other words not hires.
Another cause for concern is the complete lack of lineage of the original recording or what, if any mastering has been done. It's clear from many HD downloads an inferior source has been used. The Cream HDtracks downloads are case in point. Absolutely shocking.
HDtracks is about the best bet for quantity, the quality, as I say, is variable.