If they're on a marked peg, and the sign is on that peg (IE: the peg says "Vinty figs, $9.99", and there's a sale sign stuck to THAT peg), they owe you the price and cannot argue it. It's a mislabeled sign by the employees is all, and it may have been fixed immediately after you left (if someone being responsible brought it to a manager's attention), or it could still be that way if nobody did anything about it.