Dream-Seller via eBay offers the Crossover 40" 4K Ultra 2160p VA LED-Backlit LCD Monitor for $667.90 with free shipping. That's the lowest total price we could find by $41 and the best price we've seen for a 40" 4K monitor. It features a 3840x2160 native resolution (four times that of 1080p), LED backlight, two HDMI ports, DVI, and two DisplayPorts.

Note that delivery may take up to two weeks.