Tango uses:
Sensors
Motion tracking camera.
3D depth sensing.
Accelerometer.
Ambient light.
Barometer.
Compass.
GPS.
Gyroscope.
At least half of that is already in the phone. They might be able to manage sub-$200, maybe even approaching $100, depending on cost of sensors and what profit margin they want to build in... But $250-300 wouldn't surprise me one bit either.