There is no MSRP on violins. Just as in real estate, automobiles, or ladies of the evening, a "fair" price is what a willing and able buyer will pay, and a willing seller will accept. You were well-advised (above), when you find a violin you love, ask if the price is negotiable.
Keep in mind this tactic is most likely to be successful when you take out your checkbook to indicate seriousness of intent.