Q: Spark plug replacement on 2011 BMW 328xi

Your vehicle was equipped with 100,000 mile spark plugs. The first recommended change is at 100,000 miles. If a Diagnostic Trouble Code has led you to test the ignition system and you find that an individual plug over coil tests bad, that coil should be replaced but there is no reason, or even advantage, to replace the other five coils unless they have faults such as corrosion on the contacts. To answer your question regarding the Economics of the proposed work: spark plugs are nominal in cost. Labor time to replace all 6 plugs on this year, make and model is less than 2 hours. Even assuming all 6 coils are replaced, along with 6 new spark plugs, which again is not necessary, not at merely 62,000 miles, the price you were quoted, that is $900, is not economically rational (i.e., you are being overcharged). If you have a defective coil, replace it but nothing else need be done with the spark plugs or other coils in the absence of a specific diagnostic identifying a fault. To have a professional certified mechanic replace a plug over coil unit, simply request ignition coil replacement and this will be taken care of promptly. Please consider YourMechanic in the future for routine maintenance, such as oil/filter changes or, as well, complex repairs.