That being said, if you look at the week-to-week payoffs, there's some weird formula that NASCAR used to pay drivers. The vets, the "stars", and the past champs do get more money for comparably similar performances than do the rookies, no-names, and ne'er-do-wells. For example (totally off-the-cuff), if Tony Stewart finishes, say 12th in any race next year, he'll probably bring home as much as anybody in the top 5 if they're all 1st-3rd year drivers. I personally think it's part of rewarding the stars for the ticket sales, but I've NEVER heard about the practice being openly discussed.
There are also other monetary awards, some given to charities, that the sponsors award their drivers. Whether the driver keeps it, or gives it away, it still adds to their total winnings.