I ate at Montana a couple of weeks ago in Recoleta and thought it was quite good. However, but even better value, you can go to Yugo's Parrilla just a block away and have some awesome meat at much lower prices.

I have a 450 gram Bife de Chorizo (around 16 ounces) for a whopping 12 pesos (About $4.25). It's was the best steak I had in BA, and I've been to quite a few places.