[135iDCTFan]

My 2011 135i only has 15K miles (about 4K by me - 1K with PPK1). As the second owner (not knowing what type of gas was being run) and never getting awesome gas mileage (>14 city - and not "pushing it" by any means - 15.5 avg per computer and falling) I was thinking about having the spark plugs replaced. Other than lower than expected gas mileage, I'm not having any other issues with the car (as far as I can tell). I'll obviously try to have the dealer replace them under warranty first, but if I need to have this done professionally (not in my skills wheel house), what would you expect to pay? And would this be advisable?

Thanks-
DctFan