It's the difference between the two costs multipled by the number of people and then multiplied by the number of nights.
Since the cost of the regular dining plan varies according to season and the number of people, their ages and the number of nights in a stay differ from one person to another, the cost to upgrade is a big variable.
The regular dining plan is $45.99/night for adults and $11.99/night for children under 10. But at certain times of the year it rises to $47.99/night & $12.99/night respectively.
Quick Service is always $34.99/night for adults and $11.99/night for children under 10.