It will depend entirely on the size and position of the window and the pattern of the leadwork. Obviously a small low level window with a simple leaded design is not going to be as expensive to replace as a window 4 times as big located on a third floor gable wall with a leaded design of the family coat of arms, if you see what i mean. The best thing would be to get a few quotes from reputable companies and take it from there.
Are you sure that they really are leaded windows. Most of these are just plain double glazed units with self adhesive lead beading applied afterwards. You can get the self adhesive lead tape from any glazing suppliers (and even from some DIY sheds). While you are there, check out the cost of a replacement double glased unit to fit.
You may even be able to peel the lead off of the old window and re-use it. (with the application of a suitable adhesive of course). Bit of a long shot.
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