I paid $35 for mine, but if you belong to a local reef club I'm sure someone would have one they'd want to give away, since they come in on LR sometimes.
Aggressive in the sense that they will eat anything they can. Mine's eaten so far ghost shrimp, a damselfish, and hermit crabs (along with shrimp pellets).
However, they won't bother things they can't eat, and are generally safe with corals.