Sounds a bit steep to me!
I can't imagine that it would cost $400 for the door handle cable.
You should be able to replace just the cable, at the most the cable
and the handle.
Try getting a 2nd opinion, possibly from a body-shop. They deal with repairing and replacing car/truck doors all the time and should be able to give you a good idea what is involved.
1996 F150 Scab longbed, 5.8L, 4x4, 31x10.5 tires, 212k miles and counting. step bars, low-profile tool box, Euro-clear turn signals & headlights, Euro taillights, vent visors, class III Reese hitch, Tekonsha brake controller.
"when you have a hammer in your hand, everything looks like a nail."