There is no doubt whatsoever that one could make a low power transponder, battery powered and weighing under 2-3kg in total, which would do the job at short range (few miles) as far as TAC/TCAS systems go. Cost? Perhaps under £1000 in volume.
Then you've got the ever so slight problem of fitting the mostly-decrepit GA fleet with the other half of the system. Present cost is about £20k; this would come down but almost nobody would spend the money, and there are enough people who believe it is their god given right to not have a transponder, so nothing is ever going to happen.
That's why I would not spend money on TAS; if you are higher up there isn't normally much traffic there (other than IFR traffic playing by the rules) and lower down there are so many undetectable planes that it wouldn't make much difference.