I never really saw the prize explained anywhere, like who was eligible, and it took a surprisingly long time to find a decent link to it. But was he back solely for the money, really? Wouldn’t he have competed all season if he had found a seat?
It wasn’t really explained because Dan Wheldon was the only person who actually took up the offer. Indycar was trying to attract drivers from other series to compete in Las Vegas by offering a $5 million prize to any non-regular driver who won the race from the back of the grid. They didn’t get much interest.
I don’t think Wheldon was solely in it for the money. The challenge probably appealed to him, and he promised that if he won, he’d split the money with a fan chosen by lottery.