What changes the price
Size comes first: a tall villa sash costs more than a cottage casement. Then the number of windows, whether they are upstairs, and the material and finish. A colour or woodgrain costs more than white, and flush casements more than chamfered ones.
First-floor windows here are nearly always reached from a wheeled tower. Where a tower is needed, this may add around £150. Scaffold comes in only where a tower cannot stand, such as over a conservatory roof.
Before survey, not after
The figure you are given comes before survey, straight after the free home visit. Surveying waits until you say yes, and its job is to fix the exact sizes. Anything that must be made just for your house, an arched head or an extra-tall villa sash for instance, costs more than a stock product.
Just one window to replace? Ask anyway; a lone window simply has a call-out charge added to its price. Three windows or more and you pay the ordinary daily fitting rate.