You consume calories to power your ride. Depending on your weight and some other variables, if you ride reasonably fast, say 18 mph, you might burn 600 cal/hr, so 600 cal/18 miles. Figure out what you pay for that. If you just ate Clif or Power bars, it'd be similar in cost to the gallon of gas to drive 18 miles in a crossover SUV.
A gallon of gasoline contained the equvialent energy to 31,000 calories, so is much much cheaper than any food with respect to energy content.