Customers pay for staffs' mistakes

Wanted to buy 3 trays marked doen to R15. At the counter they said it was a mistake and tore off the same price tags from all 3 the items and told me it was over a R 100 each. Requested to speak to manager and was then told by manager it was a mistake but I need to pay the full amount. Any other store would have admitted their mistake and let me have it for the advertised price. Really iritating since all 3 items had the wrong price.