not too sure what car we are talking about here, but since most modern cars are fuel injected which uses a much more precise system for delivering and metering fuel that starting the car should not use that much gas.
another thing to consider is distance, if you start a car in the morning drive it 300 feet to the store and shut the engine off it can make the engine unhappy and could hurt it in the long run! I would leave the car running if it is still early in the morning and you just got it up to operating temperature "as long as there is another adult in the car" or if you are driving short distances between stops!
this is a generalization and may not true with ALL cars