I'm not a payment industry insider, but I made my best effort to be accurate based on my own experience with several merchant account providers and with Square.

Those were per-transaction cost comparisons using the actual rate my CC processor charges (as I said). That's interchange plus 0.04% plus $0.10, not interchange rates without markup. "Interchange plus" pricing has seemingly become pretty common the past few years, at least in what's advertised to new businesses, and now there's a couple of ISO/MSPs offering interchange plus $0.10 flat to small businesses out there, which would be even less than those costs I quoted.

That "boutique retail" store a relative of mine opened that I mentioned in another comment -- the first MSP to walk through the door to try to sell him credit card processing offered interchange-plus pricing. None of the constantly-shifting "downgrade tiers" stuff anymore. It makes cost comparisons like this possible for once.

There's still some BS costs like "statement fees" and "daily batch fees" with some companies, but that kind of thing gets overshadowed by the per-transaction savings pretty fast. If I'm overlooking something else, I'd love to know, I don't want to be giving bad advice.

> And I suspect the debit card stats are skewed by things like groceries and gas which aren't good proxies for independent retail

The study those stats came from breaks it out by merchant category. Cards were the preferred payment method at all types of retail stores they listed (grocery, department store, drug store, discount/warehouse, etc). Cash was preferred only at fast food restaurants, coffee shops and theaters. Debit was the preference over credit at all but department stores.