Really depends on the store, how long the bass has been there, and your poker face as a prospective buyer.
Does the ding bother you? If so, make it known. If not, make it known anyway. If you KNOW for a fact the bass has been there for a while, point it out. Make an offer. Be prepared to walk away if need be.
The store will probably detract $50-100 depending on how bad the ding is. Make sure that it doesn't extend to or affect the input jack itself.
Let's see...a 2008 J at a store will run for what, $1200? Offer $900 b/c of "showroom" bass status, ding, and general inability of the store to move it. See what happens
EDIT: Let it be known that I frequent the TB classifieds, and find that the some of the best "fair" deals are available there. No crazy super deals b/c people here know for the most part what's what, but no ridiculousness of $1000 SX's b/c they are "hand-made, custom basses" either.