With the bound neck and refinishing required, it won't be cheap. I'm guessing the fretboard will have to come off and be re-glued, and the binding will be an issue.
If you could split the cost, it would be one heckuva lot better for you. I agree with the comments in the other thread that the situation is suspicious, but if you never had to adjust the truss rod (I'e never touched the one in my 5123 either) then there is a possibility that the rod was bad.
If you sold the bass on an as-is basis, then I would suggest not paying for more than 50% of the cost.
"The best way to tell a lie is to tell the right amount of the truth, and then shut up." Robert A. Heinlein