The recurring JBD manufacturing costs are around $40-50 USD in lots of 500 with much of that the full pnp harness cost. No one claimed it was expensive to produce and anyone can simply add up the component costs. If you do this for a lot of your favorite electronic items you'll be surprised how inexpensive they are if you only add together raw parts. But beyond the recurring costs there are thousands and thousands of dollars of expensive engineering time designing the firmware and months of tuning time to determine the appropriate algorithms to obtain a safe, reliable, and improved 335D/X5D tuning. The box itself is just a medium to implement the tuning.

The "chip tunes" sold on eBay, etc, are generally around $5 to produce, they are sold to suckers for $20-$50, and they do absolutely nothing and include no pnp harness just two wire taps. Those are truly a resistor in a box requiring the suckered purchaser to buy in to this concept of altering the IAT signal to remap the car. Which is nonsense. In fact BMWs compare ambient temperatire to intake air temperature during cold start diagnostics so in addition to no performance you'll also get a nice check engine light and limp mode until you remove the device.

Also on a personal note I think you just don't understand much about electronics. The DME analog signals largely works on voltages not current. "Resisting" signals will not have any effect. You must remap voltages to obtain the desired results and those changes are not linear (e.g. not a simple voltage divider). In fact to avoid BMWs tuner detection system with the JBD we have to do some pretty strange things with the signal. It starts out as an OEM signal, then alters itself at times, but during other times reverts back to the stock signal. Good luck doing that with resistors.