As USB only requires that your circuit board have a card edge for the connector, and any MCU that runs fast enough can Bit Bang USB (like a $7 Parallax P8X32A Propeller) and still have enough left over for processing the G-Code and controlling the steppers and heater I do not see how it is cheaper to use WiFi. With a P8X32A you could use 2 cores for USB, 1 core for processing the G-Code, 1 core for the three steppers, 1 core for reading the sensors, and still have 3 unused cores for other stuff.

So I do not see it. I would rather have standard RS-232, for an interface, though at least USB is hardwired, and more reliable.