You might have to break your journey up a bit more than that if you want cheap-cheap. Those are huge distances to cover and you won't necessarily get a local bus for it, and certainly not one where you can also sleep comfortably on the bus. So it depends what your main priorities are.
Cordoba might be an idea on the Salta-BA route, but it really depends on your time constraints too.
You'll also find that you'll get cheaper rates once you're in the country rather than searching online beforehand. Only the "tourist class" buses have decent websites. There are local operators in the bus stations there that might get you better prices. Though I think it's also true that the buses aren't as cheap as they once were. I did Lima - Quito (around 38 hours on a bus if I remember correctly) for about $50 in 2010. The bus had cockroaches. Good times.
Enjoy it, anyway. It's a great place to overland. ^