ByT Argentina is an online agent with a wide selection of furnished condominiums that at very affordable rates (by US standards, of course).
ByT provided us with a 2BR, 1.5 bath condo with living room, dining room, kitchen, and breakfast area -- for just US$250 per week. HUGE, a whopping 100 square meters, this apartment is actually a 3BR, but the wall between the master bedroom and the living room was removed--I guess to give the illusion that the place is even bigger than it already is.
An excellent location, this seventh floor penthouse fronts Avenida de Santa Fe, half block from the Aguero station on Subte Linea D, in the posh Recoletta/Barrio Norte neighborhood. Walking distance from shopping, nightclubs, cultural sites, and fine dining. The high-end Alto Palermo Shopping Center is just four blocks away along Santa Fe, while the Recoletta Cemetary is a walkable 10 blocks north.
Minimum stay for most of the available apartments is 1 week. Long-term stays (1 month or longer) can be accomodated at even lower rates--studios as low as US$300/month. An agency fee of US$20 is actually pretty low by any standards.
A couple of drawbacks--like most rentals anywhere, they require a deposit (usually equal to the actual rent), and they only accept cash payments. However, I have used them on 3 separate occasions at 3 different locations, and I haven't been disappointed yet. You can check them out online here.