The price of food in Jiuzhaigou is relatively high due to the fact that materials and goods have to be transported from the larger towns and villages of Songpan and Chengdu. What's more, the choice of dining establishments is very limited! As a general rule, most of the hotels and hostels have decent restaurants attached and the Tibetan style ones such as those in Zehwacha stockade offer good Tibetan style food, just get ready to drink up the gallons of Yak butter tea they will offer!
Local specialties include long noodles, (lamian) which are served up in Zehwacha in a couple of noodle stores and the "Highland Barley Wine". Don't indulge too much in this or you'll be stranded the next day unable to walk!
It is a good idea to stock up on provisions either before you leave one of the bigger cities in Sichuan or at least before you enter the reserve. Water and other drinks are available in most of the villages but at inflated prices.