Simple, if you pay attention to what is known.
Here is a list of all the points which -cumulated- mean a $299 price point for the headset alone is possible (and even likely imo)
- Sony is a hardware manufacturer heavily invested in Oled screens, lenses, sensors (gyro...) and base materials (plastics, electronic components, etc)
- Sony owns the platform on which the PSVR is exclusive, this getting a cut on all software sold for it.
- Sony (Shu Yoshida) has said it will be priced to sell at cost or even a small loss*
- The spec is lesser and less costly than its competitor, let's take the Rift for comparison. The Rift has 2x Oled screens of higher resolution (1200p vs 1080p), whilethe PSVR has 1 split between both eyes.
- The Rift is $599 for a bundle which includes a controller, a camera/ sensor, a remote, a headphone (built in), and 2 games (Eve Valkyrie amongst them)
- The external box for PSVR only splits the HDMI signal and unbends it, and deals with 3d audio, with of the shelf component, thus very unlikely to be pricey at all.
*
http://www.pushsquare.com/news/2015/11/sony_not_looking_to_profit_with_playstation_vr_hardware