Are you looking for a temporary apartment, or are you trying to rent an apartment on a two year lease?
For temporary apartments, the standard is that you will pay the amount of the lease up front. And yes, depending on how long you are going to be here (the longest legal temporary lease is 6 months) it couldb e thousands of dollars they are asking.
DON'T go to a real estate agent to lease a temporary apartment. There are MANY agencies you can find online that specialize specifically in renting out temporary apartments. In fact, I have two friends that are in the business, both from the US, and could help you if you were interested in that.
If you want a two-year lease it will be very very very difficult to do. If you can even get a real estate agent to tlak to you about renting as a foreigner (not easy to do) the best you will probably get from them is "we might be able to find an owner who won't require a guarantee, but they'll want you to pay the 24 months in advance, plus damage deposits."
Thing is, the way the law is here, it is very difficult to get someone out of an apartment if they don't pay the rent. It could take a couple of years of legal crap to actually evict someone.
What owners require here is to attach a property as a guarantee against damages and not moving out or not paying the rent. The rule is usally that the property you are using must belong to a relative and be in Capital Federal, although there is some negotiating room in there. My guarantee for the house I'm renting is actually an apartment of a friend, in Cordoba.
The last apartment I was in I sublet from a friend who had to return to the US. I managed to live there just about a year. He had to pay the entire 24 months rent in advance in lieu of a guarantee and when he went back, he wanted someone to take it over to recoup the last 12 months he couldn't use.
Before that, I lived in about 8 different temporary apartments over two years.