Hi Clara,
The two biggest factors are,
1- Is the opening already framed or does the opening need to be cut into a wall and framed?
2- Is the French door a pre-hung unit or is it (2) door slabs that your installer will need to make or assemble a jamb, and then set the doors?
Assuming you have an existing opening, and it’s a standard sized opening, and the door are being set into a frame, you should expect to pay a qualified contractor (experienced carpenter) anywhere between $600.00 & $1,000.00, assuming you are providing the doors. I’m sure you could find a handy man to do it for less, but when you get into French doors, it really takes someone experienced to assure the doors will both line up, open and close properly and all the hardware is installed correctly.
I hope that helps
Joe Balich
Meridian DesignBuild