Let the seller set the initial price. Negotiate if necessary. I'd sure hate to offer 10x if 8x, or even less would buy the silver.
It may be that there are no coin shops/pawn shops/etc in your area that pay much of a price. Out of curiosity, I called around way back when silver was in the $30's and the best price anyone in my area was paying was 8x face for 90% and the spot price of silver was more than double what it is now, even with the small run up of late. If the teller wants immediate cash, at worst you should definitely get a discount under melt, which as of right now is about 11.6x face.
Unless you have a very unique item, and certainly silver quarters are not, the completed listings on ebay are the "best price" guide in existence. It shows the amount at which items are currently selling, which is exactly what anything is "worth" - regardless of what any publication tries to tell you. Keep in mind though that selling on ebay isn't free. You'll pay ~13% (haven't checked in awhile, this may no longer be completely accurate) in selling and PayPal fees as well as be out shipping expense.