Can't it just be rebuilt? I had the tranny in my 87 LeSabre rebuilt for $1500 including the R&R. It's a comparable tranny, 4 speed automatic, with a lock up torque converter but perhaps not as much electronic control. However, it's probably just as hard to remove and replace, and the time to replace the mechanicals that are out of spec are probably similar.
I would look for a family owned, non-chain, transmission shop that has been in business for a good long time, and appears to have an orderly and organized shop. (Don't expect it to be a hospital O/R, but it should be reasonably neat.)
As far as the value of the car vs the cost of the tranny. If you know the car has been taken care of, then the cheapest car to drive is the one you own.
I paid a cool grand for the Buick and didn't flinch at $1500 for a tranny rebuild. I've driven the car about 36K since the rebuild without drama.
Good luck,
TB
