Nice to see another white Cummins in the SD.
I bough mine with very similar intentions but did go with a few mods to let the engine run more efficiently. I have an AFE intake, and a full MBRP 4" exhaust and a Mileage Max Edge Tuner. I had all of these things installed the day after I bought the truck, so I can't make a valid before and after comparison, but I can get mid to low 20's on the Hwy if I am careful and 13-15 in the city which ain't bad for a 7000lb truck with MT's.
As far as diesel prices are concerned, I found that the B-20 fuel at Pearson Fuels on El Cajon was going for $4.19/gal compared to $4.39-4.50 at most other places. My truck runs a little quieter on the B-20 as well. FYI.
'07 2500 5.9L Auto QC/LB 4x4 :ARB Bumper, 12k# Warn, Yukon locking hubs, HID's, 4" Stainless MBRP, AFE Intake, Edge, Thuren Suspension, Fox 2.0's, Defiant Rock Guard, Dynatrac balljoints, Glacier 2micron fuel filtration, Nitto tires, Weathertech'd, DNX8120 Nav, Infinity Speakers, Alpine PDX5 Amp, Alpine 10" Sub, and a shovel