Re: transmission up grade??
Yes, I can see it costing that much. Here's why.
First, none of the six speed transmissions were built with a bolt pattern that will bolt to a 7.3L. You'll need custom adapters to make it fit. You'll also need to modify the trans crossmember and one or two driveshafts. That's the easy and cheap part. All of that shouldn't cost more than a grand or three.
Next, you'll need a wiring harness. You can probably get a stock one and modify it to work. Ever price a harness? With modifications to make it fit your truck, plan on another grand.
Now comes the fun part - what to do about a computer to run it? The stoock computer needs data from the engine computer to control the trans. The engine computer running your 7.3L doesn't have those outputs, nor the software that calculates what the transmission computer needs to see. The engine computer from the newer engines won't run a 7.3L, so you'll need to invent a computer to run the engine and one to run the transmission. Also, you'll need to create and program the software to control both. As far as I know nobody successfully made an aftermarket controller that could run the 5 speed TorqShift, and if they couldn't do that they sure can't make one that can run a 6 speed.
Creating the computers and software should easily eat up more than $50,000.
And that's why if you do this you will be the first person to do it.
Former Ford Automatic Transmission Engineer
“The good thing about science is that it's true whether or not you believe in it.” ― Neil deGrasse Tyson