An economy where players who have completed the career mode can buy and do anything they want, considering they've done everything that was expected of them. Before that, they receive enough to be able to progress to higher events and complete them.
I mean, it's a video game. The player achieves things, you reward them for their achievements. Make the achievements fun, and the rewards proportional to the effort the player put in. It's that simple.
Ofcourse, GT7's little tutorial mode would make a post-completion situation like that ridiculous, but not if there was a proper, long career mode that actually kept the player busy.