In looking at the Duramax/Allison combo, it looks like you will be out about 4700 to 5000 for the upgrade. Considering you will be able to resell the truck for at least that much more than a gas powered used one, not to mention the savings on fuel and longevity of the powerplant and tranny, it definitely pencils out if you can swing it.

Right off Edmunds.com for a 3500 4x4 Crew LB;

MSRP Invoice /National Base Price

$40,380 /$35,333
Select Options
PDN Heavy Duty Power Package for SRW info

$5,510 /$4,739

Understanding that you are looking at a slightly used vehicle, this info may come in handy while negotiating.

Fyi, I've had 3 Powerstrokes and had good luck with all 3. I never put high mileage on any of them, so I can't really speak to how they hold up over time.

PD