I'd say it's an ok price, but you wouldn't pay any more than that. Unless you're talking Kleins etc, old MTBs are worth very little. The rear derailleur being a Rapid Rise model is not a good point, they work ok when they are new but because the spring is doing the hard work of shifting up (instead of pulling it across to the smaller gears) sooner or later said spring will wear out and the shifting will go to hell. It also means that you can shift up multiple gears, but you can only shift down one at a time, exactly the opposite of what you usually want.