Fuel filters for Duramax's are around $35, which means if you buy one at a dealership they'll be $50. Many will change such filters every 15K miles whether they're needed or not.
I was thinking Duramax's have a 10 quart oil change, including filter. The oil filters are pretty inexpensive at $10ish outside a dealership. Diesel oils are about $1 per quart cheaper than premium auto oils, and can often be bought for around $3.50 per qt. for dino style.
I always change my own oil. Sometimes that's the only peace I have in a week's time (from the wife and two grandkids.) I'm not about to ever return any of my vehicles to any dealership service department unless the factory's paying for the repairs.