Dick, it would be a waste of public money to fit the PC9 with ADSB. They fly mainly in airspace controlled by the RAAF at PEA and ESL and the type will shortly be gone and replaced by the PC21 (presumably with ADSB).
I would imagine if it was announced that the PC9's were getting a new bit of kit a a cost of (pick a number) you would complain about that due to the proximity of their retirement.