Noise barriers are incredibly cheap. They're a tiny fraction of the cost of putting up a drywall or plaster wall. Codes say how far apart the studs should be, so you just buy a roll of batting, and unroll it into the gap between the two walls. You have to cut it to fit, but that's not a big deal.

It costs a few thousand bucks to do a whole house (during construction), even in areas with high labor costs. You can usually tack it on to the cost of insulating the exterior walls (which is basically the same process, but with a more expensive material).

Nobody does it. Not even supposedly "luxury condos". And that's assuming it's really that cheap, which it isn't because to be effective, one needs to stop all sound transmission, not just for walls, but floors too, as well as interrupting the noise transmitted through the house's wooden structure. It's quite expensive.