I'm in Argentina. I'd love to go to new beaches (not in Argentina, in south america) that are good to surf. But the cost must be low, given our cheap currency. I'm talking about costs for staying in some places, the cost of the goods (food, drinks, medical care, etc).
The actual cost of food and drink should not be that much of an issue - subsistence food is relatively cheap everywhere.
In some countries, surf culture includes staying in your camper van, or camping at the beach, so if you do want cheap, the way to go may be to club together with others and buy/hire a van or micro-bus.
Ferrugem in Brazil is popular. Haven't surfed it myself, but worth looking at - seems popular in the global surfing community.
I would recommend globalsurfers.com - they have an up to date interactive map with a forum for discussion.
South America has some amazing beaches to go surfing you might enjoy besides the ones you have in Argentina already. I am not sure what would be 'cheap' for you, you certainly will have the cost of the trip to the destination as one of the most important expenses.
Costa Rica is a good option, although is more Central than South America. You might consider Tamarindo Beach there.
Consider this article (a little bit old) from National Geographic for some interesting options.
Closer to you, you might consider Punta del Este, Uruguay once the peak season is over and the prices go down. There are some cheap Hostels there were you can stay and if you get your food from the supermarkets you will save some money. Also transportation there can be cheap in the off peak season.
Unfortunately the Argentinean Peso is currently undergoing a devaluation so it will be hard to find a place were you will be in a strong position.