Board Thread:Questions and Answers/@comment-26484276-20150610190321/@comment-5089372-20150610201836

Check out the Food Costs page, at the bottom there is a "Dragon Food Cost Patterns" section. It lists all the dragons and there food patterns. The Magic Dragon is the cheapest of course. After that Food Costs pattern D is the lowest for Purple type dragons and some of those are Aether Dragon, Aurora Dragon, Charm Dragon, Coral Dragon, Familiar Dragon, Genie Dragon, Planet Dragon, Mistmoth Dragon and Poison Dragon.

You can go through the list to see what dragons you have with the lowest food amounts and use those.