10k for one window seems a lot of money to me but, although I don't know exactly how your house is constructed, or exactly what is involved.
I have put in quite a few windows over the years in solid stone, brick or cavity wall built houses and generally it's a straight forward job.
You might have to prop up the masonry above using a strongboy or a needle and acrow props.
Then there a few different methods of doing the work. Some people cut out the hole with an angle grinder to the size and fit the window.
I usually prefer to knock out and tooth a larger hole and build the frame in as it would have been done originally.
Below is a photo of a stronboy and acrow prop.