Can anybody tell me what an approximate value for a mint '65 Chevelle/El Camino 12 bolt is?

This diff is an original with all the right numbers but it is bare.

The housing comes with the rear cover plate and carrier caps only. It is 54.25 inches flange to flange.....if that helps?

Any info would be appreciated

That would depend entirely on how bad you need it (or want it!)and how much you are willing to pay!An empty 12 bolt is going to cost you alot to build up.You have to figure on finding axles,a posi unit,gears,bearings,backing plates etc...and as for it being in mint condition,it's easy to sandblast an empty housing to look brand new.Check the wheel bearing seats for scoring and wear!
You might be better off finding a complete used diff(even a 10 bolt unless you are planning to put a 500 HP rat in front of it!)and you will probably save yourself time,aggrivation and a whole whack of money.
Hope this helps.

12 bolt rears where the thing back in the day, but now you could buy one at a junk yard complete for 75 bucks. You want strong go Dayna or im sorry to say this 10" Ford.