Mike, to install a new sliding door in the hole where an old sliding door was, I would charge about $300 in labor. These things can be a bear and they cannot be.
Now, to enlarge the hole, install kings and headers and remove the old window, do all the prep for the new sliding door, I would charge about $500 in labor. That would go up if I had to move any electrical cords up and over the header.
I would not be as expensive as the $1290, but by my standards, still not cheap.
I think that you need to shop around for three more new estimates and throw out this one.
You could, if you owned a recip. saw, do some of the work your self to work off some labor. See about this.