If you're out to get your hands on the Windows 10 Mobile-powered Alcatel Idol 4S, then Amazon is definitely worth a look. Right now, the online retailer is offering up Alcatel's latest Windows Phone for just $400. That's a full $50 off what Amazon was asking for the unlocked phone until just recently.
For your money, you'll pick up a pretty capable Windows 10 Mobile device that we came away pretty impressed with in our review. The unlocked version includes all of what you get with the T-Mobile variant, including the VR headset. Do keep in mind that the Idol 4S is a GSM-only phone. If you use AT&T or T-Mobile, it'll work fine.
While you can still get the phone for a much cheaper $288 through T-Mobile, that will require you to sign up for a plan to do so. All said, Amazon's current price is your best bet if you want to pick up a fully unlocked version of the phone at a discount.
Thanks to @thegoatishairy for the tip!