I think it's actually 10% of the "difference", not of the total amount. If I see something I might buy in any store that I've seen somewhere else for less, I just ask one of the clerks if they price match with store 'xyz' then tell them the price. Usually they just phone them up to 1) verify the price and 2) make sure they have one or more in stock.

Half of communication is listening. You can't listen with your mouth.