During San Fermín, you can go cheap or expensive. Money will not buy you fun, but it will help.
It is not easy to say if the party is expensive or not. I persoanlly believe that you don´t need much money to have fun in Pamplona. The most expensive issue you need to addreess is the housing. Hotels are not cheap. If you want a city center hotel you will have to pay around 300-500€ per night. But if you come a bit further the center, prices will go down big time. You need to understand that Pamplona is small, and when we say far from city center, it might be 15-20 minutes walk. For example I live in a neigbourhood called San Juan, across from my home there is a big hotel NH Iruña Park, wich is a 4* hotel and prices are more normal. It takes me 15 minutes to Plaza del Castillo. Beside that, our public transportation is great.
Once you take care of the housing. What about food, restaurants are normally fully booked, but there are a couple of great (cheap) Italian restaurants right at the city Center, La Tagliattella and the Saint Giovessa. Great food at great value, and servings are large. What about drinks, I recomend you when you go to bars to ask for a KACHI (it sounds like “catch it”) it is a large drink, you can order a Kachi of beer, or Sangría….. and share it with friends. It is much cheaper that asking for 4 Beers. One tip, Sangría is over priced, I recomend a PAmplona favourite, it is called Calimotxo, it is red wine and Coca-cola.
Here you have an article published by the Chicago Tribune about high end San Fermín.