What probably screwed you is the "Semana Santa" period. Some airlines really increase their fares during such holidays. But the $699 fare return is very much publicized in newspapers, so probably you could have gotten such fare for other dates.
Have a great trip to Argentina and post a trip report when you return so that we can all read about the new AR
service. Tell us how long the stopover in LIM
is, whether you need to leave the aircraft at LIM
and, of course, tell us all about the A310, about the service, the meals, the seat pitch, the IFE, the beauty of the Argentine flight attendants (my God are Argentine women beautiful!!!) and any other details you can provide.
Next flights: MEX-LAX AM 738, LAX-PVG DL 77L, SHA-PEK CA 789, PEK-PVG CA A332, PVG-ORD MU 77W, ORD-MEX AM 738