do the parents have papers showing they are pure bred? do the parents have their hips/eblows tested showing they dont have bad hips? 500 is CHEAP!
i would go see the parents. are they calm and confident around you? are they skittish/shy?
if none of that matters to you then 500 is a good price. i would choose wisely though because you have to live with this dog for 10+ years. if you get one with bad nerves or bad hips you'll regret not spending the extra 500 in the beginning on a better pup.