Popular airlines Spirit in the US and Ryanair in Europe have been gaining millions in ad revenue for years by renting empty spaces on and in their aircraft. How does that work? Simple, they set a price and say "Hey, if you pay us $14 million we'll put your logo on the outside of all of our aircraft for a year." They even sell space inside the cabin. According to USA Today they charge "$196,000 for three months of ads on the overhead bins in Spirit's planes, $119,000 for ads on the tray tables or $18,500 for ads on air-sickness bags." The airlines get extra money and the advertisers get more exposure. The problem is that all those ads annoy passengers who don't like being inundated with ads while inside of a plane: one of the last 'safe havens' from the outside world. So while airlines and ad companies are happy, the customers are not.
Selling ad space on and in aircraft may or may not work out because the customers don't like being inundated on planes, but what about on spacecraft? While NASA might have nearly insurmountable restrictions on spacecraft advertising, what about the private industry? What is to stop SpaceX, Virgin Galactic, or any other private space company from renting space on, or in their spacecraft?
How many people follow every space launch? How much exposure and publicity would a company get if they had their logo on the outside of a spacecraft? How much would a company pay for to advertise on one launch? In order to find out we can look back at past advertisements in space.