In short the individual cost is insignificant compared with the results of a collision.
Unfortunately that is exactly the CAA's line on cost justification.
True the cost of a collision with a commercial airliner is measured in mega-bucks. But even the best kit hasn't stopped commercial jets hitting each other or the ground.
The Mode S consultation has continally foundered on the cost-benefit analysis question and the line from the CAA is 'isn't your life worth £1500?
This has been used to support the carriage of ELTs (not PLBs though) as well as Mode S. It is a classic politician's trick and should never be used by forumites.....
The reality is that for some transponder-equipage can be around £5k upwards although usually it is about £3k, when fitting is easy. Add to that the cost of a useful TCAS, not the cheap ones, and we are talking a lot of bucks. For a lot of the Permit, BGA and BMAA fleet that equates to 40-75% of the cost of the aircraft itself. How often do you buy a piece of kit so relatively expensive - except perhaps an engine once every 10 years or so?
Added to that we are looking at a piece of kit that benefits other people, not necessarily the operator. Effectively, you may be asking for me to lash out cash that I can ill afford, on something that does me little or no good, so that some of you can tear through Class G with your eyes wide shut, relying on the voices in your head....
If I'm not going to fit a transponder, like large numbers of my colleagues, what possible use is TCAS/PCAS to me or anyone else, come to that?