More often than not NA prices are cheaper due to no VAT unlike everything here in the Uk and the EU. Though it's swings and roundabouts as some stuff is cheaper in the UK, sometimes EU prices are cheaper Drizz usually hooks people up with cheaper Brazil deals as does Salsa sometimes plus I'm sure for you in Argentina has some stuff cheaper than elsewhere and then there's region specific subs like that Mafia II dlc. steamprices.com has a top bargains, top ripoffs section to compare prices between NA, UK and EU and showing the differing percentages.
Brasil will always be the best place to buy because they use local currency, that being said I pay less for 2K, R* and Valve games (but still in US dollars.)