Yes, you can pay to upgrade, just remember that you must do it no later than 72 hours before your arrival date.
The cost will depend on where and when you are staying.
If you are staying at a value resort, you will receive the quick service plan free.
To upgrade to the deluxe dining plan it will cost you $40 per adult per night and $12 per child per night.
If you are staying at a moderate or deluxe resort you will receive the basic dining plan free. To upgrade to the deluxe dining plan it will cost $30 per adult per night and $10 per child per night, unless you are staying between December 17-31, when the upgrade will cost $25 per adult per night and $9 per child per night due to the higher seasonal value of the basic dining plan.