This http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=9762&page=1#Item_3 thread on how much should they cost:

I have had two quotes ranging from 6k-9k and I am not sure how much these are and how to compare the two?

I’ve asked a couple of people in the trade what they should cost, and have been told c. £5,500 for one panel and a 250l cylinder

certainly don’t pay over £5k for a single panel and 250l tank fully installed.

TBH I don’t really see why it should need to cost more than around £3.5-4k if you deal with an installation company instead of a sales company, …… with the commission structure.

One thing though is I think there’s probably a temporary shortage of people with FGas certificates along with the other qualifications, for installing this in line with the regulations, which is probably keeping some competition out of the market. If you need a hot water solution now then fair enough, if not then it’s probably better to wait a few months for the dust to settle.