It depends on the sizes and colour... double or triple glaze... In the City or country...
To give you an idea, We're going for white, 3 bedroom semi detached, 5 big windows, 3 small, 1 frontdoor, frenchdoors, vistaterm elite, our quote is all in for around 6k
A less pricey solution would be to keep the window frames and to change only the glass for more energy efficient glass, I know someone got his whole house done for 1500 euro.
And yes it would make a big difference as nowadays, the technology is affordable and much more efficient than 21 years ago...
Also, if the house is cold, it would be worth looking at attic, wall insulation.