PLease do not think that I do not think what you did was impressive and cool.
I guess I see what you are saying on cost. Since you already payed for it I guess it is cheaper. If you bought a whole car. But you do not have to do that unless you want to try and install the whole system from that car.

A shifter costs about 50.00 my system costs 1200.00. That can be used with any ecu. And gear ratio. To install it takes about 3-4 hours. If the transmission is in the car already. Set up in the software takes about 30 minutes tops. Then maybe an hour or two to drive around and set shift points. Maybe an hour or two to fine tune some shifts for an application like this.
Wiring up a factory ecu to work is about 10 wires as well. Still needs to be done regardless of which way you go with the transmission.

The factory stuff requires and entire other car to be disassembled and the wiring removed. The differential needs to be removed and or replaced and in his situation new axles that bolt on so tone rings can be installed. Maybe a custom differential for the correct gear ratio. And maybe custom mounting brackets like yours to mount a differential that did not belong in the car. Just removing all the stuff to get to and the wiring from the donor car would take at least a day. If you sold all those parts for enough to cover just your labor time for removing them and posting and selling them, then you could count that time as free.
Now you have to install it in the new car. Lets say another day if you are moving.

But you are right you could buy a whole E or c or something that has everything you need for a couple of thousand dollars. But if you need a new key or any new parts for anything that has to do with the DAS system ever. guess what. You have to buy a whole car again or find someone parting one that you can get those as a set. Or make a whole new set. None of it is cheap.

It is not like I do not know how long it takes to work on these cars. I have been doing it every day for 20 years. I have taken countless cars completely apart for various reasons. And it takes a huge amount of time. And I have work instructions for anything I do not already know how to do at a couple of clicks on a key board.. And any tools that anyone could possibly need.