Is there a standard way to come up with a reasonable price for used electronics?

A guy I work with wants to buy my first generation DROBO (basically a RAID style hard drive enclosure). The problem is, I’m not sure what to offer it to him for. Up here, its selling new for $350. Is there some sort of equation for taking the current new value of an electronic device and coming up with a reasonable price for it after x months of use?