Since Petra is worth exploring for a full day, you have to eat there. You can always take a sandwich and have one of the best picnics you'll ever remember. But, if you decide to eat there, there's not much to choose from: There is a buffet, typically found in this country, for 17 dinars (about 20 euros). There is typical Arabian food to fill you to the top. It's not cheap, but it's really good. It's an option.