San Fermin can empty your wallet very quickly.  If you want to save money during the festival, do most of your eating and drinking from goods at the grocery store and hit the bars away from the most popular areas.  To get an idea of what you'll probably spend each day, here are some suggested budgets. 

Cheap Budget:

Breakfast 3€
Lunch 5€
Dinner 5€
Drinks 12€
2 Pinchos 5€
A ride on the Ferris Wheel 3€

Total: 33 euros.

Not As Cheap Budget:

Breakfast 4€
Lunch 10€
Dinner 10€
Drinks 25€
Pinchos 7€
Amusement rides and candy 7€
Bullfight ticket at the stadium 20€

Total: 63-83 euros.