In the real world, you would need to wait till you have enough money to buy something. Of course, this don't apply to rich people as they can buy anything they want since they got the money. This also applies to racers as they can't just buy any upgrade since they need to have more than the amount needed to buy upgrades for their car.
Fortunately, in this game, you don't need to have lots of money as you can win every upgrade. Drive your car as fast as you can till you reach the finish line. The prize reward depends upon on your position in the race.