The average price for external repairs is normally £150 per window
The worst case price for full rebuilds is normally £300 and that is still more than 1/3 of the cost of replacing them!!
Cill Fronts From £50>£130
Small Resin Repairs £8
Large Resin Repairs £60
Box sash Splices From £15>£50
Glass 4mm From £30
Dryseal (putty) £7 per meter
Painting From £45 per M2 (two coats)