I had a chat with an audiologist on my mother's behalf recently. From what I understand, a good deal of the price discrepancy between certain models is not down to how well they work, so much as the physical appearance and how discreet they are, ie almost invisible to readily visible. Apparently the NHS issued hearing aids are pretty good, but just a bit on the big side.

Like any other high cost items it would definitely pay to shop around. Lots of these guys are on the internet now so wouldn't take long to get some prices to compare.