First off: I made the mistake of leaving my bike outside this weekend, in New England. Don't all jump on me at once: my college expressly prohibits taking bikes into your dorm room. Something about a fire hazard. But yes, not the brightest idea.
So. My rear derailer is back to jumping and clutching and "floating", not grabbing the gear, and shifting like s***. I took it in for an adjustment, and it runs better, but still like c***. It no longer switches gears without me doing anything, but it still jumps, clutches, and floats. (LBS in fact warned me it would do this). But I can't take this. I'm told that my rear derailer... well, the teeth are worn down, as well as rusty, and the chain probably needs replacing. My bike is a 1998 (or '99) Specialized Hardrock, which is slightly too small for me, but still quite sturdy. Is it worth it to just outright replace the derailer and chain altogether? How much would it run me? I can't afford a new bike, or even a used one.