Changing flat window to patio doors.........How much ???

[nutcracker]

We have been thinking of changing our flat window in the dining room too patio doors/french windows, but haven't the faintest idea how much it should cost.

My dad could do it for us, but he isn't always well enough so i'd like some idea of how much it would be should we have to pay to get it done.

Any ideas ??

Someone's dp does windows - is it zephyrcat?

We paid about £850 to have our door with 2 windowns at the side ripped out and replaced with patio doors.