I know what you mean. I have two of the later 54-56 boxes, one in my 27 and one for my 29 rpu someday project. Years ago I tore the one for the 27 down to rebuild it and found the worm shaft was bad. That was only a hundred bucks or so, but if the pinion shaft was bad that would have been in the $ 400 range, and that was 20 years ago prices.

I think you might find that the current going rate for these is somewhere in the range you quoted, and being truck boxes they were subjected to less than careful use and good roads, so they can be a little sloppy internally. Start hitting the swap meets too.

Good luck.

Don