There is no "mid-grade" premium here. We have ONE gas station within 45 miles, and its only premium is regularly $1 a gallon more than the cheapest gas is. Right now, regular is $1.67; premium is $2.69. If it averages 20 MPG, that's an additional nickel a mile.
Nickles add up when you're putting several thousand miles a month on a car.
Thanks, all, for the input. You've helped me tremendously with clarifying this.
I fear the purchase price is, indeed, the tip of the iceberg. I do have recent experience with vehicles that get low mileage, and the adverse effect on profitability is startling.