I'm traveling around South America for a year, and staying a month in each big city. It is cheaper to stay a whole month and cheaper the longer you stay.
There are plenty of sites catering to us gringos. I avoided them.
Rental contracts here are usually 2 years, sometimes 1 year.
Alquiler temporario can mean anything for days to 6 months. Ask them if they rent for your specified time.
Go to the clasificado section of the daily newspapers - online versions
They mostly have ads from imobiliarias which have commision charges.
inmuebles.clarin.com Alquiler temporario
zonaprop.com.ar Alquiler temporario
Craigslist.com for buenos aires - you can find owners and they don't have comision fees which are usually 10% or higher.
These sites include a ton of ads. It took me 10 days to search through, map out the ones I wanted, get replies to the ads, (better yet call them, but if you don't speak Spanish get a friend to call for you), and go see the apartments.
I saw 10 apartments under $800 dollars -all furnished studios or 1-bed in areas of Palermo, Recoleta and Abasto. They all had a/c and wifi included. I chose only the ones that included all the bills. Three of them had a pool. I chose one only to be told to wait two days and then the owner cancelled on me. Expect some flakey owners.
The one I finally got was a brand new furnished studio, bedroom area, dining table with four chairs and a nice equipped kitchen. A/C, cable, wifi 3M, all bills included, taxes (abl), gastos comunes (building fees) for $650 a month. It was through a rental agency and they charged 10% of the contract, and we had to pay a month's deposit as well (returnable). 10th floor, balcony and pool. There's a subway stop four blocks away and a Disco supermarket 2 blocks away.
You can find something affordable. I saw furnished apts downtown for $450 and they were kinda grubby looking.
Make sure you ask them if bills are included, and if not how much they usually cost. Bring your laptop to check the wifi. Make sure the faucets work, and check the stove. Walk around the block to get to know the area. How close is a market? Etc.
Warning: One ad we found on Craiglist had photos that were NOT for the advertised apartment.
Good luck. Also, learn Spanish!