You don't notice knocks or pings because the knock sensor detects the knocking and retards the timing to prevent it. The timing is not the only thing that gets retarded, your power and fuel mileage suffer from this as well.
By using premium or mid grade the timing will be restored and fuel mileage and performance will be restored to some level. Remember, these are high compression engines, most of them 10:1.
It would take some scientific testing to determine the correlation of price per gallon versus change in fuel mileage. I expect that it would be cheaper to run regular, but I expect that with retarded timing, you may be asking for a carboned up engine.