Early December is getting into high season for travel to Brazil and Argentina, but $7,000 RT from NYC would probably be one of the cheaper D fares as I would expect an I fare to be around $5,500 and of course Z lower still. $13,000 should be C or J, where the latter is last seat availability. Also December 7th is now less than 28 days away, so missing the advanced purchase deadline is likely to make tickets more expensive.

However, are these OW tickets? That can make prices much higher. The day of the week can also matter but only by a couple hundred dollars IME.