As a rough guide I charge £1 per window/door; large or small; it averages out. An 'average' semi (if there is such a beast) with 9 to 11 windows and two doors, would be about £10 or £11. If there's a 'standard sized' conservatory attached, I'd add another £3 to £5 on. Bay windows would be extra too. For a double breasted bay window detached house I'd be starting about £20.
But other factors affecting price is location, frequency, type of windows and access issues.