You get what you pay for as with anything.
You could find a cheap torque converter and shift kit anywhere from $500 to $1500.
Expect to pay $3-$3500 for a pretty good automatic. One that will work fine for 95% of the buying public and performance crowd. This includes a few added clutches, better frictions, pistons, band strut, etc.
But as we push the limits harder and harder, obviously more stuff needs replacing.
Another $500-$1000 will get you some much better internal parts.
(cryo treatment, new shafts, etc)
If you have the money and want the best in performance and durability, you could spend up to $6500-$7500.
Top of the line racing trannies have hours of time in the machine shop replacing thrust washers with thrust bearings and improving lube passages and many other tricks. The diesel transmission industry hasn't gotten to that point quite yet (a few have), but is quickly approaching. I'm scared to find out how much these will cost.
-Chris