The first thing you need to know about eating in Pamplona during the festival is that they raise the prices on the menus. In fact, many print new special menus with "different" prices just for the festival.
Bocadillos are common and may be your best bet throughout the festival. They are filling and cheap. Bocadillos is Spanish for "sandwhiches". They fall under the category of food called "pinxtos", also known as "tapas". In English, pinxtos are appetizers. Bocadillos are sandwhiches served in the form of stiff baguettes with sandwhich fillings such as egg omelette, ham, beef, or fish. They only cost approximately 2€.
The choice of foods in Pamplona are not great in variety. You bounce from one restaurant to the next and you'll find the same old menu items. At the end of the day, look for a cheaper restaurant, because the menus are all the same. Often the menu will be posted in the window.
Exceptions to the lack of variety are the few following: Sobu (below), a trendy Japanese restaurant across the street from Hotel Tres Reyes, has Asian food and it is really good. You will find some donor kebab restaurants and a few pizza places, but that's it aside from the same thing over and over.
If you are looking for restaurants along the bull run, you will find them to be packed. It is better to walk down some other avenues to quieter streets that will be likely serving the same thing. Below is a picture of a bar/restaurant on Estafeta:
Any questions? Please post your questions in the comments area, and I will be happy to answer you.