Rich,
I have to agree with Shamus about the value of brass locomotives. The value never goes down. I think the best thing I can do with mine is get a piece of Truscale track for them and plexiglass display cases. It doesn't do any good keeping it in the box.
What it really is good for is adding to subject matter to a photograph. You can't tell from a still photo that it runs like crap!
Now here's a catch 22. What do you do if the only way you can procure your favourite all time locomotive is by purchasing a brass one, as it's unavailable in plastic? Gee, er, um, I suppose you will bite your cheek until you taste blood and make the purchase with passion. Nothing wrong with that, just don't expect it to outperform your Kato drives!
I mentioned rolling stock. If you get some brass passenger cars or freight cars that are professionally painted, you can't beat their appearance. The detailing is top notch and not as prone to breaking as the high end Proto series plastic freight car details are, if mishandled.
I have some brass passenger cars I have yet to have time to paint, but I have a wide vision caboose and an 85' auto parts car a friend gave me because it was too big for his layout. When offered monetary compensation, he told me not to worry, as he scored it used at a show for US$20!
Despite the prices you see, you can find some great values. Forget the locomotives, but don't rule out brass rolling stock!
George.