Chuck,
Certainly you are correct, assuming you make $10,000 either way. But, what if one is more profitable than the other? I don't think anyone knows the answer to that question right now. Also, if we take costs out that customers are willing to take on themselves then the picture changes. Lots of customers seem very interested in sanding the cars themselves to save money. The cars I have seen are quite nice as most guys want to do a sweet job on their ride--which, of course, makes me happy.
The picture really changes if you can make and sell 4 cars for 10,000 profit in 1/2 the time it takes you to make and sell 1 car for 10,000 profit. (I am using your numbers and they are just for conversation--as in not real.)
Always experimenting.
David