What makes a dog that yips or barks on track worth any amount of $ ?

I have to agree with Buck. Ever seen a nervous herd? How do they know there is a predator close by... do they smell it, hear it, do they see the vague and shadowy movement of the predator stalking ever closer through the brush... all of the above? Will we ever really know 100%... I doubt it. But the point is that sometimes they know, because the just know. And sometimes they just don't know and you can come in guns blazing... lights, sirens and all, and they'd just sit there and want to fight. Ive seen 100% silent coyotes and feral dogs run deer to death... they never opened up, and the deer was always way ahead of them, but the deer KNEW that dog was after them. How did they know... I don't know. To make things even more confusing, I've seen fully open hound dogs bay-up bedded hogs. I can't explain it, but it happens.

I've hunted fully open, fully closed, and everything in between. My preference... the dog that puts me on game... plain and simple.

Any dog that consistently puts you on game is worth the money you paid for it.