0 to take a particular amount away from a price: --
1 to take a particular amount away from a price, value, etc.: --
The supermarket joined the fuel price war yesterday by knocking 5 cents a liter off its petrol and diesel prices.
Customers can knock hundreds of dollars off their energy bills by following three simple steps.
Living next door to an empty home can also knock 10% off the value of your own property, say experts.