McDonald's is quite different than in the USA, since cows are  fed grass, not grain.  Plus some of them have the adorable McCafe coffee bar.

The McMagnifico or Quarter Pounder meal is A$11. 

Other cheap eats options include pretty good pizza, sandwiches or empanadas. There are lots of street vendors and little cafes where you can find a low-cost breakfast or lunch. E.g. cafe "Dulcinea" on Santa Fe Ave. offers a lunch of toasts, chese, jam, cream, fruit salad, fresh juice and coffee for 15 AR$.

If you prefer to sit down for some international cuisine, you can find Chinese, Thai, Middle Eastern or Japanese restaurants that will cost you just a little more.