Also, let's say I go for the rebuild, why does it cost 2000$.
This is not the same as replacing the whole transmission.
Why do they say 2000$ if they don't know what the problem is until
they actually open/disassemble it?
Sometimes you can find a mechanic who will do it for substantially less.
A rebuild entails dropping out the tranny or transaxle, disassembling it
by piece, cleaning it to almost surgical conditions, identifying the bad
replacing the worn or unserviceable parts, reassembling the entire system
some of these reassembly steps require measurements and adjustments as you
go), reinstallation, filling with fresh fluid, test the work, adjust or
repair if not
working within specs, and delivery to the customer.
You must make enough profit to justify the time you spend, your education,
the special tools that are required to do it, put beans on the table, and to
the job in case you have a return under warranty.
These systems are complex. You ought to see one torn down. You wouldnt
believe how many parts, big and small, are involved.
Sure, you can go to a junkyard and buy a used tranny. In many cases, that
very good solution... In some cases, it can be disappointing.