I had an MP15-22, generally a good gun. Mine went back as it had reliability issues, came back shooting anything like a dream as far as cycling went.
I was hoping to have very good accuracy, it was ok for a plinker but not a good target option (some are, most are ok and some are plain bad).
I would check Gun Broker for a used one.
$500 is high, you should be able to get one in the low 400s and used around 350-400