"raw" VR data is about 10 megabytes a second of black and white infrared video.
plus audio and position data of either controllers or hands.
After its been processed it can give you a sparse pointcloud of your living room (which is not overly bad) but thats not something thats done by facebook, yet. With a bit more work you could get a dense pointcloud.
Unlike AR which will need a bunch more processing on device to get position and geo data, VR is mostly just slam
Facebook generated $29 per user in the US last year. A small bit of extra info about people's gaming habits is not going to improve ad targetting enough to make up for a $100 gap in price.
But it's clear from the reviews the $100 was mainly achieved by cost-cutting.
They may still lose some money on it but that's just an investment in becoming market leader in VR.
First it measures your fitness level. Those devices have high quality acceleration and gyroscope sensors. They could know your visual and acoustic acuity.
They measure what games you play, for how long, at what time.
With these data and processing done by AI created by engineers they can extract and know more about yourself than you do.
We know from Snowden that places like the NSA collect and save data expecting technological changes in 5-10 years in the future for processing that data. The technology for processing does not need to be ready yet.
This is extremely useful for Advertisers and secret services. This is not what you say you are, this is what you are.