It is 300 pesos. You will have to get there an extra hour beforehand. It took me about an hour to pay the fine. You'll have to make your way to the migraciones window in the airport, and then they will take your passport and give you a piece of paper to go pay the fine at the Banco de la Nacion in the airport.
I waited for 45 minutes in line at the bank and there was only 6 or 7 people in front of me. But it seems that the branch may be the last place that Argentines, with a valid plane ticket and permission from AFIP, can exchange pesos for dollars. The bank only has space for two tellers, and it seemed that they don't really want more than one teller there.
Be sure to get there early, and to pay the fine before you try to check in with the airline, otherwise, you'll end up wasting a lot of time.