The trouble with the off-breed dressage horse is that you’re in the same price bracket as the young-warmblood-with-potential. And people are, generally, more likely to value potential (the dream of winning Nationals) vs. proven (and less likely to be regionally/nationally competitive)-- even if their own riding is nowhere near regionally/nationally competitive.
So to command higher prices, the off-breed horse has to be a) carrying a star-studded resume of good scores at the level the horse is advertised as doing, and/or b) be a good color/have other breed-show benefits.
That said, there have been some absolutely insane prices for honestly just decent rideable horses (like this auction group). So honestly who the hell knows.
As an aside, I am a mid-career professional and have evolved from riding those WBs with potential in college/early 20s to my current guy, a 9yo QH gelding with the proverbial heart of gold. And for me, right now, he’s invaluable: soaks up training like a sponge, tries hard every day, and has 3 decent gaits that can be jazzed up to 7s, super pleasant on the ground and under saddle, but can tolerate somewhat inconsistent riding schedules when real life intervenes.