Totally agree, it will probably be cheaper for them just to get the window replaced.
We started on one a couple of years ago, it was a big mansion that had it stuck on the inside, I think it was to make it into safety glass as it had been a care home at one time.
Anyway we did one set of patio doors, 2 of us and it took the best part of 4 hours and 3 cans of solvent cleaner. Didn't do any more after that!
Even if you are working at £20/hr that is still £160 for one window, and that's not including your matierials, they'll not pay it.