Cost would be negligible since some receivers are already transmitting anyway
Ok this time I looked this up. Mode S is 1030 Mhz, or 1.3 GHz. That is way different than 2.4 Ghz But, I admit that I forgot we were now transmitting at such high frequency (geez above the space band!). But I don't think it will be the frequency but the power that is the issue. Besides even if I am wrong, nothing is cheap in aviation. Even hand held transceivers cost more than equivalent ground based radios. And $2,000 aviation transponders would likely cost $200 if they were for civilian use.