Industry standard here is that if an incorrect price is scanned, the customer gets the item for free or a $10 discount, whichever is less.
We don't go by an industry standard here in MI--we have a "label law." Customer, by law, is entitled to ten times the difference in a ticketing error with a minimum payout of $1 and a max of $5 per error. Item must be paid for--if you bring the error to the attention of the cashier while the transaction is in process, and the error is corrected before you pay, you get zilch. The law does not apply to sale signs or shelf labels, only labels affixed to an item.
Just a bit of FYI