To rent an apartment on that budget you'll need a guarantia (title from another property that is put for collateral for the rent contract). I'm assuming you don't have one. That means you'll be paying expat rates in either inflated Pesos or in Dollars. The current exchange rate for Dollars is about 1:12. That means you have less than U$300.
I really don't think you'll find anything for that.
Sorry.
GS