And you could buy a car off the showroom similiar to what they were racing that weekend.... Hence the saying, "win on Sunday, sell on Monday."
In the Petty Blue video, they interview Lee Petty and he says, "I can buy a new Plymouth off of the showroom for $800 and be able to win a race with it that weekend." He's talking about the mid 50's when Nascar and Daytona were first starting out.