Hi and welcome to GT Planet!
After finishing certain series, your price car would be one of the 4, and ALWAYS random. So to get the prize car you want, you do the series over and over again until you get it, or you cheat:
Save at the end of race 9. Enter race 10, hit start, drife for a few seconds and exit the race. If your advantage in points over the 2nd place is enough, you'll win the series and so a prize car! if you want to "change " the car, reload the savegame and try again 👍
Good luck