Cheapest is by Bus. A very comfortabe but long (18 hour) trip will take you to Puerto Iguazú from the Retiro Bus Terminal in Buenos Aires. It will cost you about 400 pesos for a return trip (thats about 125 USD).
Flights are more expensive. Aeroparque - Iguazú (and back) will cost you at least 250USD.
We went there and spent Christmas visiting the falls. We flew and it is just 1 hour 45 minutes! I recommend taking the boat BUT you will get very very wet so be prepared. It was great! That was our second time there...this trip we were showing it to a friend from Colombia who was visiting for the holidays. Have fun!Lee