In our lab (i am a grad student working on a UAV-related research project) we are actually using UAT ADS-B transceiver for one of our UAVs.
According to FAA, UAT 978 MHZ devices should be a must by 2020 on all GA AC (and hopefully drones <-- that's just my ignorant hope). It is not that expensive, honestly. The cheapest solution so far for full In/Out capability is around 2400 bucks. I think it will drop even further. It uses the same message standards as a normal 1070 ADS-B solution would, it just has a different frequency.
The UAT that we are using is of the size of a box 7 in x 5 in x 2 in - it is actually kind of light. You would think that it should be heavier). I mean, there is definitely a way to make it much smaller so that it could be put on much smaller UAVs.
I think there should be that and heavy fines as a nice addition - those photos from that UAV hovering near the threshold are really scary...
