Hi, we used Triangle Fire Systems, and found them to be good. I can't say how much it would cost because it really depends on how many floors you need to install sprinklers in, and how big your house is. One thing I would say, though, is that you should make sure that you cater for additional costs such as electrician's costs and any carpentry/building additional costs because these won't be covered by the sprinkler company.
We did a lot of research on different companies, and out of the ones that seemed to be accredited by the relevant safety bodies, Triangle were the cheapest and most responsive. There were a few that were much cheaper, mind you - we just weren't sure how trustworthy they were.
Hope that helps.
http://www.trianglesprinklersystems.co.uk/