Despite its history of immigration and a highly heterogeneous society, Argentina is a fairly protectionist country. It's a member of the regional customs union MERCOSUR, and goods imported from outside the union face a common external tariff. The country had a trade surplus last year equivalent to 5% of exports (source), and its GDP is growing around 7-8%. This is against a backdrop of rising commodity prices (a boon for exports), a monetary and economic collapse only ten years ago, and high inflation (which reduces actual income growth).
Food, however, is not expensive compared to what we're used to. Steph calculated that produce in the supermarkets costs between US$.50-$2.00 per pound, cheaper than the range in Boston. But the selection is not what you'd find at Stop 'N Shop. It's hard to find limes, and out-of-season produce like watermelon is nowhere. The "international" aisle at Shaws had ten varieties each of salsa and tortilla chips; here the cuisine is more influenced by Spain and Italy, and Mexican ingredients are harder to come by. The stores are smaller, and food comes in smaller packages; the largest milk container in the supermarket is a 1-liter (1.05 quart) carton.
On the other hand, the supermarkets all carry huge selections of Argentine wines, beers, and imported liquors. Alcohol consumption in Argentina is generally low, by cultural convention, so there's none of the American puritanism imposed on the aisles.
So far, we haven't lacked for anything we need in terms of food. Maybe if we tried to find special Middle Eastern spices, we'd have to venture farther afield than the local supermarket. But the restaurants here are superb and cover a very broad international spectrum, which makes me think everything is sold somewhere if you know where to look.