Read these 2 messages from the bottom up to this one:
To be honest, the reason high performance transmission cost so much is not really due to the parts. The bulk of the cost is the labor. For me personally, at this point in time, I can tear down a 4L80E, inspect the entire thing, clean it, re-inspect, stack the internals on the bench (we do it to double check our work, just have a 4L80E yoke welded to the table and stack it from the output shaft up), then put it in the case and assemble the internals and the valve body assembly ready to be on the dyno, in about 4 hours if everything goes smoothly. When I first did it it took me 2 days, and that was with people guiding me and watching. The labor and the knowledge/development is what makes these bitches cost so damn much.