I sold one at the Ford swap meet rebuilt, Richmond 3.73 Trac Lok, new lines and hoses. $500. I picked it up for $125 out of the junk yard years ago. Guy a few aisles over was asking $900 for a rusty seized one. Swapping in an IRS and the 9 was getting in the way. So, depends on where, when and who is selling it. I've seen them average around $400 stock around here.
I've seen people take housings where perch distance was off by an inch or more and because they could get the perch over the spring pin (because leaves without the axle flop around) and get the hardware cinched down they saw it as a bolt in. So you may need to relocate the perches.
There is the E-Brake, I'm pretty sure the cable wouldn't be a direct swap, though I'm pretty sure you can get a cable that would be a bolt it (Not like your doing this to a Mustang II).
You may need to get a larger volume master and install (a) residual pressure valve(s).
And finally brake bias. That can be handled by resizing brake components front and/or rear or installing an adjustable proportioning valve.
And depending on your level of anality, if your going from an 8" with 2 7/8 tubes tot he 9" with 3" tubes you may want to get different spring plates. I'm not sure about this one because it's been years (decades (ouch)) since I owned the 67 and 71.
Around here, a complete stock one in all good useable condition goes for about $500. If it has a traction lok, add $200-300 depending on the ratio. Maake sure it's all there and all good. Parts for these are a little hard to find and expensive. Also, be sure to look into the E-brake issue. Many people report problems getting the E-brake to work properly.
For that kind of money Currie would send you a 4-pinion Traction-Lok™ in a carrier with your choice of gears, ready to bolt into your existing housing. With the right tools, a couple of hours and you're off and running.
i sold mine a couple years ago for right around $1000 but it was rebuilt with all new bearings and seals, rebuilt calipers, new rotors and a 3.70 geared open center section also with new bearings and seals, it was also painted and detailed correctly for installation into a mustang as well. hell of a lot cheaper than buying a currie rear even if you had a trac-lok installed.