In the Interac model, Interac charges the acquirer less than a cent to process that transaction from the merchants into the system. We also charge the issuer the same flat fee amount to process the transaction on the other side into the account to ensure that the money is available, etc. Those are the only fees that are charged within the Interac debit model. I can't comment on the Visa model.
On June 16th, 2009. See this statement in context.