Normally, technology items (keydoards, recording gear, etc.) have the lowest margin of any music gear. Why? Because technology changes - the companies have more R&D costs and they try to "set price" more to maintain the perceived value of the gear. Also, the stores don't buy as much quantity (too expensive and too much potential for new and better gear coming out).
It's very common that the margin is only 20%-40%. Given that that a retail operation probably needs about 30% to cover overhead (and make some profit) there would normally be only about 10% to bargain with, and possibly not any bargain room at all. In the $500 range you probably won't have much room to bargain.