In-store only and with activation of a qualifying service, Costco offer its members the Apple iPhone 8 64GB Smartphone for T-Mobile for $499.99. (It's not available online.) That's the lowest price we've seen for a new iPhone 8 by $199. (For further comparison, Apple charges the same for a refurb, although it is unlocked.)
Also available is the Apple iPhone 8 Plus 64GB Smartphone for T-Mobile for $599.99. That's another all-time price low for a new unit. Both deals end November 15.
With these companies, it might be easier than you think to negotiate your monthly bill down.