thanks all very useful.
i think im too cheap!!!basically, i restore sash windows, bringing them up to 'as good as new', that involves replacing any rotten members, reglazing with 'thermo' glass and fitting draft strips. most of my work is to listed or heritage buildings, and i way exceed the remit. i figure that the windows are either restored or replaced and replacing 270yr old sash windows is gonna cost! ive been charging 12.50, im going to charge £18, i can restore most sash windows in around 30hrs so that would put an 8ft x 3 1/2 ft window at around £750 (including materials).
does that sound reasonable to you?