Someone posted the payout for a 16-driver field for all 3 race lengths, but for the life of me, I can't seem to find it now (if someone could send me the forum link, I would appreciate it). Assuming they're all consistent, I'm fairly certain the 20-lap races have the highest payout per minute. Remember to take into account the time in between races to host, add drivers, then load the track for shorter races (for example, you'd have to add the time setting up a new race by 4 if you're comparing a 5-minute races to a 20-minute race).
edit: after further consideration, all the races may even out in a very general sense in terms of payout once you factor in loading times. There will obviously be slight differences, but I don't think it's anything dramatic.
I also found the post I was looking for, and based on those numbers (and my earlier ones) the average payout for a 16-driver, 20 min.race is 10,695 CR; a 7-driver field is 7,950 CR. So the bigger the field, the better. In a 7-driver field, the top prize is the same as 6th place in a 16-driver field, and sequentially works its way down from there.