If it is a 3 series posi, the axles are still good and the housing is mostly clean and unpitted, then 13 really isn't too far off for a good starting point imop.
Although, about 16 years ago I bought all that for $400. Drove across to Tonawanda NY. Guard had a look and just waved me through on the way back. Ahh, the old days!
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65 Laurentian post, 67 Grande Parisienne 4 door HT.
Most of my B body 12 bolts didn't cost me a cent except for the gas to go get them, because they were liberated from 65-70 B body Chevys and Pontiacs abandoned in the woods. There were lots more at one site than I liberated, but when I went back a few years later to get the rest, somebody II was told the the stock car guys but I can't verify this) had gotten to them. What had been complete units were now stripped of all their components, with only the cases left behind. I missed 2 particularly good ones in this manner but at the same time I'm thankful for the ones I did get.