knock sth off (sth)
phrasal verb with knock verb [ T ] uk/nɒk/ us
informal to take a particular amount away from a price, value, etc.:
Living next door to an empty home can also knock 10% off the value of your own property, say experts.
Customers can knock hundreds of dollars off their energy bills by following three simple steps.
The supermarket joined the fuel price war yesterday by knocking 5 cents a liter off its petrol and diesel prices.