The cost to have an automatic transmission rebuilt depends on the transmission and vehicle and whether the transmission shop needs to remove the transmission and replace the transmission themselves or whether you bring the transmission by itself into the transmission shop to be rebuilt.
I took an automatic transmission into the transmission shop and the transmission shop rebuilt the transmission to like new for $400.00 which was very reasonable.
Although I've seen some transmission shops advertising lately that they can rebuild some transmissions for $200.00 or less which is a good deal.
You can sometimes buy a transmission rebuild kit for $150.00 or so but it's a hard job of tearing down an automatic transmission and putting all the new parts into place.
So you can sometimes have the transmission rebuilt for $800.00 or even $1,500.00 or even $2,000.00 which sometimes includes removing the transmission and replacing the transmission after rebuilt.
You can also look online and purchase a rebuilt transmission for your vehicle for about the cost of having your current transmission rebuilt.