It really does depend on alot of aspects. If the windows werent too bad and you can tell they will come clean without too much effort then i would charge a building that size about 500 pounds but if the insides are as bad as you make out with all those leaded if they are leaded inside aswell then i will throw another ton on top maybe 2.
I will always have a figure in mind, on this occassion 600 pounds but i will always price well over with room for negotiation, you have to sell it.
how far over the amount in your head depends on the size of your balls, if you got big balls you would go all out and quote a grand if medium size balls then 800. For me it would also depend on if the owner or whoever pays you is gonna be there the whole time, they wont like it if it only takes half a day or even a full day people like to get the most out of you for as little as possible. ive lost jobs cos the customer sits down works out how long it took me to clean his house and realises per hour its more than he earns and hes a qualified joiner... muppet. i think it was more the fact hes wife took a shine to me!!!