A dealer unit is the Mercedes of rebuilt transmissions.
Adsit - not a lowest-cost supplier - has them for $1395, and you would probably get hit for $200 to ship the rebuilt unit and get the core back to them. 8 hours for labor is about right, which you should be able to get for $600 at an indie. So that's $2200 plus or minus. If there is a rebuilder in your area, you might be able to save the freight.
You need to measure spending that against getting $1500 or so for the car and buying something else. If you think the car with a new transmission will get you through 4-5 years, and if you like the car, it's not such a bad deal.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe