There are many forms of prizes you can win, these are GT Credits, A-Spec experience, B-Spec experience and cars. They are won in all events, but each event handles the prize differently.
A-Spec and B-Spec racesEdit
Both A-Spec and B-Spec races reward you with credits and exp. at a set rate that never alters, but should you complete an entire section (such as the Sunday cup) with all gold trophies you will be rewarded with a car, but unlike the other Gran Turismo games you cannot win any prize car more than once, this means that keeping hold of won cars is now more important than any Gran Turismo before.
In any special event you can win credits and exp. at a set rate, but if you re-race that same event with the same position(3rd or silver etc.) you will not get the credits again, but you will recieve the experience. In order to get more credits you must do better that you previously have in that event.
In bonus races you can win both credits and exp. but you can only win the credits and experience once unless you do better than previous attempts.