That seems a little high for just a rear main seal replacement. I had my 2001 Dodge 1 ton Cummins rear seal changed when I had a new clutch installed. My labor bill was a little over $500 including the new clutch replacement, and transmission refill.
Yours should amount to transmission/drive shaft removal, torque converter removal, and then remove/replace the crankshaft seal. Once it's on the lift, an hour or two should do it. Of course there may be extenuating circumstances with access to the necessary areas.