Most times it's cheaper to take your vehicle to a transmission shop and have your current transmission rebuilt instead of replacing it with an already rebuilt transmission.
Transmission you buy to replace your transmission are usually just rebuilt transmissions anyway instead of new transmissions.
There's a transmission shop here who only charged $200.00 to rebuild a transmission for my truck and then charged another $500.00 to put the transmission back in so I paid only $700.00 to have the transmission pulled out, rebuilt and then put back in which was a good cost.
An already rebuilt transmission would've cost me $1,500.00 just for the automatic transmission and that didn't include putting it in the truck.