One of my clients has an Android app that I need to do testing on time to time. I wanted to get a modern Android phone that ran a stock version (vs one with "customizations") without spending a ton of money. The recently released BLU R1 HD was the perfect fit. For $109.99, you get Android 6.0 with 2 GB of ram and 16 GB of built in storage (can expand with microSD to 64 GB).

While it's probably not the best phone on the market, it's unlocked and can be used with just about any carrier. You can buy it on Amazon for $109.99. If you have a Prime subscription, you can get a discount if it's bundled with lock screen ads.