Right, so buses are basically free as mentioned in a previous post, but knowing how to use the bus is a different story. There are different fares for different distances and you have to indicate what fair you're going to pay to the bus driver when you hop on board. The only fares that I've paid for are 1.10, 1.20, and 1.25 pesos. You can tell the driver whatever fare you want, but I think as long as you pay 1.25 you won't have any issues traveling anywhere.
I'm only inlcuding the following info in this forum because I wish I knew this information when I first came to BA....ALSO, the Guia T, which you can pick up at any News Stand, will show you which buses to take and where they go. Once you figure out the bus system, I guarantee you'll cut your cab usage down by 98% and save a ton of pesos/guita/mangos/plata! Good luck and have fun!!