How much do people usually get paid to fly a plane with an ad attached to the back?

The banner towing questions have been addressed but I will confirm that the Cessna 172's and 182 in which I have flown most definitely have keys. The key controls access to the magnetos, without which the engine won't start. Position one activates one set of magnetos, position 2 is the other set and position 3 is both. Just like in a car, to start the engine you turn past the last position to engage the starter motor. Once the engine is running you release the key and springs return it to the "both" position.