Update to help people doing this in the future --

The AA RTW agents did have the itinerary priced the next morning when I called (overnight but less than 24 hours). The "US price" was significantly higher than what the online planner tool had given me, so I asked them to reprice it at the "Argentina price". This took another day. This time the price was what I saw in the planner. So the online tool is probably giving the price as if the ticket is bought in the country where the trip starts.

I'm now working to purchase the ticket in Argentina with a travel agent there.