You guys are approaching this all wrong. Look at it from the customer's point of view.
They don't give a rat's behind if you're a pro or not. You have cool photographs of their car, that's all they care about. I know a guy who attends MMX (dirt bike) events and takes pics of everyone. He makes prints right there on-site and then walks around to find the drivers in his photos. He sells 4x6 prints for $5 a pop, and you know what? They eat them up like candy.
These guys have a lot of time, and money for that matter, wrapped up in their car/bike/whatever. They are just thrilled that someone else shares their passion and is out there taking photos of them doing it. It makes them feel like pros themselves to have cool pictures doing what they love to do. They'll show their friends and hang your prints in their garage. If they can afford to race, trust me, they won't scoff at a few bucks for high quality photos.