I had no trouble to get a D class fare on lan.com when selecting chile as my country.

Tarifa (Fare) $629.000
Tasas y/o impuestos (Taxes) 49.710
Sobrecargos (Surcharge) $20.000
Descuento 3% LAN.com (lan.com discount) $-18.870
Total $ 639.840

I have two advises that might help you:

1. Log on with chile as your country and try to get the 3% discount
2. There is an offer by mail where you can upgrade any SCL-IPC-SCL fare for 10.000 lanpass kms roundtrip. According to the rule is only available to those who received it by mail, but give it a try. You have to first buyyour ticket on lan.com and then call +5625262000 option 4 and ask for the UPG. The lowest Y-class fare when logged in Chile's page is in O class for $229.000 (about 420 USD) plus taxes

Good luck!