Adorama via eBay offers the Dell 28" 2160p 4K LED-Backlit LCD Ultra HD Monitor, model no. S2817Q, for $399.99 with free shipping. That's $104 under our mention from two weeks ago, which was bundled with a $150 gift card, and the lowest up front price we've seen. (It's a current best by $45, outside of the mention below.) This tilt-adjustable monitor features a 3840x2160 (4K) native resolution, 2ms response time, dual 9-watt integrated speakers, USB 3.0, DisplayPort, and two HDMI inputs.
A close price: Dell Home, via its Member Purchase Program, has it for $408.49 with free shipping.