For comparison sake, my cost per mile, diesel/Lucas is about $0.26. That's the truck in my sig, driving very conservatively with the latest hand-calculated (and adjusted for slightly oversized tires) figures. I'm probably running a little bit more cheaply than that, but I don't have any more recent numbers (the last tank had a low-pressure fuel pump change about 75 miles into it, so I lost some fuel to that, throwing my numbers off a bit). Mileage figured @ 17.9 mpg....at this level it appears there is a small savings of about 1.5 to 2 cents per mile. (I dont have hand calculated mialage numbers for these driving conditions on Diesel only?)
Using the numbers you've got above, you're running about $0.28/mile with propane.