in gas prices.
15 mile detour = 30 mile round trip. Assuming 20MPG, that's 1.5 gallons of gas at $3.50/gallon, or $5.25 more. So as long as you are filling up at least 5.25 gallons at $1 cheaper, you still come out ahead.
Unless you also include the time value of money for driving 30 miles to get gas.