Interchange fees have been in the news a lot lately, but they're notoriously difficult to put an exact figure on because PIN-based debit incurs a flat per-transaction fee, while signature debit is generally percentage based, and varies with the merchant in question. (eg, Gas stations will pay less interchange than Best Buy or online merchants.)

When you see figures in news stories, there are a lot of assumptions going on behind the scenes; eg, they'll typically assume a $40 transaction, aggregate PIN and signature based debit, are based on broad-industry studies, &c.

There is also variation depending on what debit networks are being used. If you look at the back of your debit card (especially if it's from a smaller regional bank or CU), you'll see many more logos than just Mastercard or Visa.

I thought many places here (Canada) accepted debit but not credit because it was a fixed fee per transaction not a %

Debit cards issued by Visa or Mastercard and operating over those networks incur a lower fee than credit cards but follow the same interchange fee structure. Australian Visa interchange fees are here: http://bit.ly/visainterchangefees. Just incase anyone was interested. Whether or not these cards require a PIN or signature depends on whether they use the Chip-and-PIN system, which all new cards issued in Australia should.