Seeing as it would not require an additional Teensy, the price should be less than half of the price of a whole Ergodox.
how will you possibly get this to work without a teensy?!
You would not. Misterjoe got it wrong. The keyboard was designed for having a Teensy on the right side, and a simpler, cheaper I/O expander on the left side. The I/O expander is pretty dumb, it can not replace a microcontroller.
But because the PCB is symmetric, it is possible to switch the Teensy and the I/O expander.
All the components on the left side would be as on a right-handed keyboard, just on the opposite side of the PCB and with the Teensy upside-down.
The case would also have to be made from right-handed case parts.
In other words, this would have to actually be a right-hand kit - but you would build it left-handed.
I think it would be better to call it a "one-hand kit".
It would
not be cheaper than half an ErgoDox, because a Teensy is more expensive than the MCP23018, a TRRS socket and the cable combined. There are also a few resistors and LEDs on the Teensy's side.
What would you do with the unused TRRS socket? Keep the socket, put something in the hole, or let there be a hole?
You would also have to modify the firmware.
This idea has been put forward before, so I think someone else might already have built his own left-handed ErgoDox with firmware.