The average price for a 60×90 UPVC double glazed window is around £350-£500. For bigger 90×120 windows, the average is around £525-£750.
Factors such as where you live in the country and which floor the window is on will affect the price.
If you are looking for 4 small windows for an apartment, you can expect to pay between £1,400-£2,000.
If you are looking for 12 windows for a 3 bedroom house, with a mix of window sizes the cost is likely to be between £5,250-£7,500.