If you run it again and improve your time, you get more money and more XP, but it's incremental in relation to your improvement. Improve by a tenth and the payout is relatively low, improve by a second or two and it's much higher.
Another added bonus is, and this is something we could use in GT as well, is at the end of each week when the new challenges come out, you get additional bonus money based on where you finished in relation to the community at large in the previous week's challenges. The higher you finish on average the bigger the payout. Gives you a real incentive to actually do each event and to push for better times.