One of the most common ways for a transmission shop to sell major work is the use the RDI method (Remove Dismantle, and Inspect) of selling. Simply put, you are told that your transmission has an internal problem and the transmission needs to be removed from the vehicle and disassembled to determine the extent of damage and thus the cost of repair. You are not given any indication of a cost of repair until the transmission has been torn down and is in a thousand pieces. What if the price is more than you are willing to spend on this repair? What if you would rather sell the vehicle rather than spend to repair cost? It's a little late now because you will have to spend the quoted amount for the internal inspection if you decide not to have the repair. If you want the transmission to be reinstalled in the vehicle, there is usually an additional expense and the transmission will probably not even work.
Transmission professionals are supposed to be expects in the diagnosis and repair of transmissions. Most shops have been in business for years and should be able to give you at least a range of costs that a major job will run before the transmission has been removed. After all, they have presumably worked on many transmissions.