A good rule of thumb for South America is people are cheap and ( particularly imported generally) things are expensive.
In argentina locally produced products are cheaper but usually of lower quality. Things like applicances are very expensive. Our 5 year old Kenmore side by side frig sold for $1000 dollars. In the states you can buy it new for around $800. Some models of cars in Argentina sell for more used than new(the government sets the price on new, too low, so none are available).
Soulskier is right that wine and meat are cheaper but you can expect other costs except energy to be about the same as in the states. According to Mercer Consulting data, which I find suspect, the cost of living in B.A. is about 110% more than what it would cost in an average U.S. city.
In any event its not a bargin like it was a few years ago. However, if you are from someplace like California or Manhattan it will seem cheaper for you.
Gasoline, natural gas, and diesel are cheaper in Argentina but are subsidized by the government. According to the news today the government in Argentina is cutting significantly energy subsidies for 2009 so I don't know how much they will cost in the future.
By the way Soulskier, your comment on mid priced cities in the U.S. sounds more than slightly arrogant and insulting, maybe you can clarify.