If you look at recent sales on eBay you'll see 530i E34s selling for 3.5-4.5K for ones in good condition and 125K-150K miles. Exceptional ones can go for as high as 7.5K, but that's rare. A rebuilt tranny is in the 3.5K range. If you need one (assuming the worst), that falls in line with the $500 to $1K estimates, providing someone wants the hassle.
It makes sense to see what it will take to fix you car before you decide what's reasonable to let it go for.
Z