The main types of replacement windows are:
Casement windows would be the lowest priced windows of the four above styles. Sash windows, especially timber sash windows, are likely to be the costliest of the four.
Choice of construction material.
You should also know which type of material window you prefer as this will influence the cost.
Of the above, you can find very cheap softwood windows, but the cost of maintenance is comparatively high and lifespan comparatively low. UPVC is most often the lowest cost with Aluminium & hardwood significantly higher priced.
SO, once you have a rough idea, you should go online to a comparison website like www.upvcwindowsfitted.co.uk where you can find more detailed information about all types of replacement windows & get free quotes.
That will be a good start in order to learn “How to get the lowest Double Glazing Price online”.