Pre-Owned iPhone 6s $199
The iPhone 6s is the best cheap iPhone with a 4.7-inch display that you can buy in 2019. The iPhone 6s offers a better experience than the iPhone 6, and it's not that much more expensive when you explore pre-owned models at Gazelle and other retailers.
Amazon offers a $199 iPhone 6S that works on Simple Mobile and you can buy an unlocked iPhone 6s that is renewed on Amazon for $153.
You can buy a used iPhone 6s at Gazelle for $189 and you can get an unlocked model for $229. This means you can use it on almost carrier. Gazelle sells versions specific to AT&T, Sprint, T-Mobile, and Verizon if you prefer to go that route.
With a pre-owned phone from Gazelle, you get a phone that's undergone a 30 point inspection and a 30-day return window if you run into any problems. You cannot buy AppleCare+ for these. You can do a monthly payment plan with a Gazelle partner if you prefer to go that route. Here's a look at buying Gazelle Certified phones.
The iPhone 6s includes a 4.7-inch Retina Display, 12 MP camera, support for Apple Pay, the Apple Watch and includes support for 3D Touch, which you won't find on the iPhone 6 and iPhone SE.