People complain about the price of gas, but you might be surprised to learn that fueling with coffee can be four times as expensive as buying a gallon of gas. These examples show the costs of 12 ounces of coffee. Some were brewed at home; others were bought already brewed. Prices for tested products are averages. For Dunkin’ Donuts, we used the prices of small and medium cups to get the average cost for 12 ounces. That would add up to $18.03 per gallon. Your money will go a lot further with home brews.