Here is a recent response on cost to install a sliding glass door which is quite a bit easier than a french door but same general size.
Turning windows into french doors cost.
The patio door after it has been installed.
Turning a window into a patio door may seem like an overwhelming project for a homeowner.
Andersen windows and doors sells french hinged and gliding patio doors in an array of materials like wood composite vinyl and aluminum.
You can expect an average cost of 525 and up.
The good news about turning a window into a door assuming it s wide enough is that the advanced carpentry required for altering a load bearing wall.
There s also another option you might not have considered yet converting an existing window into a french glass door.
The cost to install french doors starts at about 400 and with labor estimates running between 75 and 180 per door.
Because you are almost certainly going to be increasing the width of the opening and very likely raising and lengthening the support header over the window as well you are going to to be above the cost in the referenced comments probably in the 2000.