I just booked a 3 day, 2 night package through http://www.artourism.com/ for my wife and me $614 (US) each to stay at the Sheraton. It includes airfare from Buenos Aires on Aerolineas Argentinas, tours on both sides, and a boat trip.

We're going in July, so I can't comment on their service until I get back. They're very pleasant on the phone, and very responsive.