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Hey Big Dave,
I'm driving a truck similar to what you're looking for: 2006 F350 4x4, Long Bed, Crew Cab, Automatic Transmission, single rear wheel, w/ 4.10 rear end. Except for running 80psi in the tires, it's all stock with 36k miles. With stock air pressures in the tires, and "normal" driving habits, I was getting <12mpg city, and 13.8mpg hiway. I aired up the tires to 80psi (sidewall max), installed a ScanGaugeII, and started paying attention. I do some coasting in neutral with the engine running, some DFCO. I now easily get >17mpg city, and >23mpg hwy.
My FE would be a lot better but I drive a LOT of short trips, and a lot of the time the engine is cold. My truck gets 20-50% worse FE when the engine is cold. Also, my house is at the top of a pretty good hill with a 0.4mile long steep gravel driveway. Going up that driveway a few times a day at 5mph really kills the mileage. And climbing the hill at the bottom of the driveway when the engine is cold kills it too...
Today I had a best-ever day, with a 34.6 miles country-road round trip that averaged 27.2mpg. I had to re-run the same trip later in the day, and got 26.4mpg (I drove faster on outbound leg of the second trip because I was late). Average speed was 42 mph the first time, 43 mph the second time. Slowest speed was stopped =), lowest speed limit was 30mph, highest was 55mph, 90% of the route was 45mph speed limit.
No aero, no mods (except for tire pressure and ScanGaugeII)... 27mpg in a 8,000 lb. 4x4 truck. I think your 20mpg estimate is low, and your $8k estimate is high.
Also, trucks with the 7.3 liter engine like yours seem to get better mileage with load than the newer 6.0 or 6.4 liter engines do. Towing my 10,400 lb. trailer (very tall and square) in the very hilly terrain in Oregon gets me ~9mpg. I don't know that running a big overdrive gear would get the same good results that you're getting with yours. On mine the FE plummets when the LOD gets above 50% (on the ScanGauge).
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