Pricing is very subjective. It depends a lot on your area, the coverage, the design, artist's experience and reputation, their past experience with you, their mood, etc., etc. I wouldn't have a problem asking them (very nicely and non-judgementally) to explain the price and say I thought it would be less. A decent artist shouldn't have a problem explaining that to the person paying for their service.
At the end of the day, you have 2 choices. Pay their price or go to someone else. You're not shopping for a can of soup.