We just went through this on Wes's '98 this summer.
A used tranny will run about $800. That's just a junk yard tranny, not a remanufactured or rebuilt. Then, unless you can do it yourself, there is labor for taking the old tranny out and putting the new tranny in. This takes a lot of time to expect it to cost a lot. We found it was going to be about $1200 to put a junk yard tranny in. It was about $2500 for a new OEM tranny and about $3200 for a jasper tranny. The Jasper trannies have all the TSB's handled plus they just all out build them better and you get a 3 yr / 75K warrenty. We went with the Jasper.
The transmissions Torque converted is gone ( or so i think, i get the flashing OD lite) Gf bottomed the taurus out and punctured the tranmission pan drainning the fluid and kept going for about 2 days till it was completly gone. They replaced the pan and refilled it with fluid and it worked for about 2 weeks. Now it just stalls out when i put it in drive or reverse unless i feather it with some gas. I think rebuilding the transmission wouldn't work because its been fried.
No offense to you Matthew if you work there or know people that do, but Aamco has a pretty bad reputation. Heard lots of horror stories about them and Cottman, despite their warranty. Now since it's a chain I'm sure there are probably exceptions (as in individual locations that are good), but I would not let them touch my car.
I repaced 2 transmissions this year. The first was in a '92 vulcan(160,000) that cost me $200 from a private individual. Rock thru the oil pan on the donner car. The second was in a '93 essex(blew the head gasket yesterday at 107,000) because of a collapsed cooler. This one cost me $450 at a wrecking yard, but it had 20,000 on a rebuild with the documentation to prove it. I do all my own work which is good because the second one was a '92 going into a '93 so I had to modify the wiring where it entered the tranny instead of buying the ford adapter for $100.