Hi,
Important thing is to shop around for quotes that are based on someone viewing your actual windows, their sizing and any potential issues/fitting concerns etc.
there's too many people who quote over the phone, you get happy and then when you want to go ahead, they increase the price etc.
I had crazy variations for the conservatory we made early 06 - varied from around £5500 supplied & Fitted to well over £14,000!! B&Q came in at £12,000!
We went with a local independant supplier and although there were some worrying issues for us on completion time, we had a brilliant conservatory for around £7800 with every last detail finished.
Another similar type of place came out to quote us on replacement windows in white uPVC and for 5 windows (one of which is a tricky square not rounded bay) we were quoted around the £1800 all fitted.
There's plenty around here, I'm not sure where you are though!
hope this helps
DtM! West London & Slough UK!