Ah, I hadn't seen a DNF before and didn't realize the race could end before everyone got to the finish line. With the way it's set up for the chocobos to continue running towards the finish line non-stop (albeit at varying speeds), I'd assumed all races would wait for everyone to get there, since it's guaranteed that everyone eventually would. Ok, now that I understand that part, I agree that it shouldn't provide a reward. It should be something like:

DNF: forfeit entire entry fee. No EXP
Last place but finishing: forfeit a portion of the entry fee, but your bird does gain EXP
Seventh place: Get back your entry fee (and perhaps a few MGP more, but not much), gain EXP
Sixth and up earn increasingly higher MGP prizes, especially for first place.