Certified ADS-B transponders run multiple thousands of dollars, but as with everything in aviation, much of that cost comes not from the product but from the certification process. The hardware itself is not necessarily expensive. Proving to the FAA that the hardware is safe and reliable, and maintaining insurance coverage for when NTSB inevitably cites the device as a contributing factor to an accident, is what incurs the expense for manufacturers and why the prices are so high.
There are pilots building battery powered homebrew ADS-B receivers out of a Raspberry Pi and a USB software-defined radio tuner. The whole setup runs around $120. They aren't FAA certified, of course, but are not required to be since all they do is receive. It wouldn't be difficult to turn this design into a transponder with little additional weight. Surely it's conceivable to manufacture something even lighter and smaller and less expensive when done on a large scale, if the certification requirements were set reasonably enough to make such a unit commercially viable for consumer level "drones."