Citizens from some countries have to pay a reciprocity fee (tasa de reciprocidad) before arriving in Argentina; ideally you'll be reminded of this when you buy your airplane ticket. This fee is equal to what Argentines are charged for visas to visit those countries. You'll need to pay this fee online via credit card; see www.migraciones.gov.ar/accesibleingles and click on 'Reciprocity Fee.'
Fees are US$78 for Canadians (good until a month before your passport expires). Check current regulations as rules can change quickly.