B&H Photo Video offers the Samsung HMD Odyssey+ Windows Mixed Reality Headset for $299.99 with free shipping. (Samsung charges the same.) That's tied with our February mention and the lowest price we could find by $97. Features include:
  • 2880x1600 AMOLED with 3.5" diagonal per eye and 110° field of view
  • 6-axis accelerometer and gyro plus a 3-axis compass
  • AKG headphones with 360° spatial sound
  • access to Windows Cortana
Update: Microsoft Store now offers it for $299 with free shipping.

Update 2: It's now out of stock at Microsoft, however it's still available at B&H Photo Video and Samsung.