Tridents were not so popular when they came out. Seemed people could not tune them, hard starting and all the other junk that something new brings out in the tinkerers. As far as current prices for the Trident, I am all ways happy when I find something I like that does not have a big following because it will be priced much lower than the hot thing of the time. Yamaha just seemed to kill the UK motorcycle market in my openion.
My limited qualifications are that I am a former (has-been) 1967 TR6 mist metalic green owner. Bought it used, never had a problem with it, only fun and sold it for the same price I paid for it ~$800 in 1970.