I don't think your method is valid because it does not account for differences in the efficiency of the electric vs. gasoline motor at converting their respective energy sources into mechanical work.

Edit: Changed my mind, I think your approach is fine because the MPGe numbers do take into account the efficiency of the electric drivetrain. I come at it a different way below but get pretty much the same result. So yeah, I think you are right, it's $10-$11 bucks to drive 300 miles.

That's the bad news. The good news is the electric appears to be somewhat less expensive to operate than your estimate. The Smart has a 17.6Kwh battery pack, which is good for a 68 mile average range. At $0.11 per Kwh, it costs $1.94 to charge the smart for 68 miles worth of fun (?). At that rate, you are looking at $8.56 to drive 300 miles. Charging is not going to be 100% efficient, so you will have to put in more electricity than you get back, but even with 15-20% losses you are still looking at about 10 bucks to drive 300 miles.

Some time ago I crunched the numbers on the Tesla sedan and came up with similarly impressive results. For what it costs to charge the big 250 mile battery you can purchase enough premium to drive your S550 about 45 miles.