I am going to give you the amount of food that you will need to get your dragons to level 30.
GRAND TOTAL: 4,204,601 Food PER dragon to level 30.
Food Farm Guide
There are 3 Food Farms you can build in this game.
Depending on how active you are, growing the 30 sec food yields the best gold/food ratio. However, it requires constant clicking which can annoy some players. The best one for the later stages are the 12 hour Rainbow Flower. They are the most cost efficient food at the later stages of the game.
This chart is taken from bbg.