The cheaper price you see advertised in the U.S. is AT&T's subsidized or discounted price with AT&T paying the difference to Apple on a buyer's behalf, which requires the buyer signing a two year contact with AT&T.
The iPhone is sold in the U.S. as carrier locked with AT&T only regardless the price paid. The iPhone is not sold in the U.S. as officially unlocked. Even if it were, support and warranty coverage is honored in the country where the iPhone was sold as new officially by Apple. You should purchase an iPhone in Argentina where you will have support and warranty coverage in the event your iPhone needs to be exchanged under warranty during the first year after the purchase.
