Matt, for my old AF stuff, there's tons of track and roadbed still around. Even at 50+ years old, it's still about $2.50/length plus shipping for the rubber roadbed, or $3.50/length in the LHS. For a good-sized layout, that's around $240. In return, you get sound-deadening and a realistic look that's not so realistic-looking. Neat, cookie-cutter mounds under your tracks does not have the variation normal in railroad track; moreover, the first thing people do to try to improve on it is add ballast to cover it up, again. Tankist gave you a very good, concise list of what the pros and cons are. Personally, I like the sound of trains and don't like the look of that rubber stuff. The cork looks nice, but too neat and regular for me. I imagine that, when I finally get to throw mine together, I'll put down sheet-cork cut to length and shape. The advantage is that the cork glues well to the foam base, and the track to the cork. After that, ballast that will have the irregular, sloppy look of a conventional train's bed.
An afterthought...I'll probably try to stain the cork dark brown with an irregular black smear down the middle. Old earth, and trains drip oil. I'll probably drizzle a lil black stain down the middle of the ballast for the same reason.