Back in December of last year. Cricket Wireless, an AT&T owned MVNO, was offering iPhone 7's for US$149.00.
There were, of course, conditions. The phone was locked to Cricket, it could not be used on any other network at time of purchase. After six months you could have it unlocked. But it would only work on AT&T and MVNO's running on AT&T's network. It might have also worked on T-Mobile.
The point of this is that Apple uses its supply channel partners to offer iPhones at every pricing tier.
That's how I got my SE. They were cleaning up stock. I paid $50 USD for it and had to wait six months to carrier unlock it. That was a year and a half ago mind you.