I've heard varying reports that profit margins on iPhone are as little as 3% for retailers other than Apple themselves. This would mean the price retailers, even large retailers like Best Buy and Walmart, are paying for the iPhone is somewhere around $600+ for a base iPhone 7. This is why you rarely see the iPhone go on sale, unless it is a discounted installment price through a carrier (who make much more profit off their service than they do selling you the phone)
You can find "new" iPhones for sale cheaper online, but never through reputable retailers that are Apple Authorized Resellers. I think I'd trust Craigslist before a shady unauthorized online retailer.