Sprint offers the 64 GB iPhone 8, preowned, for $ 6 per month when combined with a Sprint 18 month Flex lease. The offer is valid from 5/17/19 to 5/23/19. Upgrade to a 256GB model for $ 16.67 per month.
The fine print: "Own the iPhone 8 64GB $ 6 / mo. After $ 8.59 / mo credit, used within 2 bills. With approved credit, 18 month lease and new service. If you cancel early, remaining remaining balance. "
More details are available on Sprint's website.