$400 for labor and parts?
$400 just in parts?
Rebuild, or repair?
Repair is open it, replace only the bad reverse parts, close it up while ignoring the other worn items inside the trans since they won't (can't) be looked at.
Rebuild is open it, completely strip it down, clean everything, inspect the rest of the gears, shafts, bearings, hubs, etc. inside the trans, replace the known bad reverse parts, then close it up after assembly.
Obviously, repair is easier than rebuild, but not as complete or thorough.
For a rebuild, parts included? Very cheap... suspect in fact.
For a rebuild, labor only? Probably a little high on average, but it depends on how well and thorough the work is done. I know of someone who charges $500 labor only for a rebuild. But it is very quality work
For a repair... probably about right to open it, put a reverse idler in it, and close it back up again, without taking it apart further.