Banco Nacion is the best "legal" exchange rate that I´ve found so far in BUE, and I went to several places. As previously said "Ezeiza Taxis" (white and blue sign, right in from of you when going out on the arrivals hall) are the cheapest (about 90$) and they have an offer of 65$ for going back to the airport. Be sure to take their phone number when you get there.