(Deep intake of breath through teeth) You'll need to talk to a joiner and discuss in a lot more detail. There are so many variables the price could be anything from a fair bit to quite a lot.
As a very rough idea, reckon on £350 to £600 a window, but as has been mentioned, much depends on where you live, and whether you want hardwood or softwood, metal or nylon pulleys, fitments and catches included etc etc. Quotes vary enormously - we had a set made up in West Wales which worked out at less than half the quote we had from a London joiner!
And don't forget the VAT - most quotes will be excluding VAT.