Synth prices rose over the years, all the 6 series went up in price. On top of that the Trigon 6 had more packed in (3 oscs and other things) so was priced a bit higher than the others at launch. AND it's not been out anywhere near as long (P5 is coming up to a decade in the wild now!).
What I can say is MY TRIGON 6 has become my favourite of ALL Sequentials, not just the 6 series (had 'em all), it even made me sell my Prophet 10 r4 as I simply prefer the sound of the T6 and then you get to all that extra power and sound shaping with 4 and 2 pole filter, third OSC, Feedback and drive... the layout is awesome, best of the 6 series. The knobs feel the best (they loosen up after some use and feel beautiful and never wobbly or thin like on the other 2).
T6 packs a hell of a punch, it's has everything I'd hoped my P6 would have way back (I recall being very let down with the P6's tone it's the worst of the bunch tone wise but obv it's flexible but to me nowhere near as flexible as the Trigon once you learn it well). T6 is also, by far, the best "mono synth" in unison mode (even just 1 voice). It can approach sounds of the other 2 and do even more, OB-6 obviously has its own special character and beauty, nothing sounds like that, but P6 has always sounded a bit generic to me. T6 fixed that.
It's cool it's a secret of sorts, gives us more chances to be unique in our music.