Yea, some of the more unusual prototypes or rolling stock are available only in brass, and then only in very limited runs.. These are also very labor-intensive to produce, and all these factors combine to make brass models quite expensive.
Caveat emptor though.. Brass doesn't always mean it will run well. There are brass models that look great but run like tired dogs.. Then there are brass locos that run well.
Also some have operating issues... For instance, I'm not sure if it was Overland, but I remember a few years ago one of the brass importers was offering an HO SD70ACe loco... It uses a drivetrain mounted in the hollow fuel tank, with driveshafts going out of the fuel tank ends to the trucks. This means the trucks don't swivel more than a couple of degrees, consequently the engine won't work on anything less than 42" radius curves!
I think I'll pass...
It's a crapshoot..