I would say that it's really a marketing ploy
For years there have been loss-leaders, a product that is sold below or at cost to attract attention to a brand, they way that OnePlus is are operating with the 'invite' system is a little like this. We've all seen adverts where they advertise a car that is far cheaper than it should be then in the small print, "only one at this price" and it is guaranteed that the vehicle is not on the lot and you are told that it has already sold but while you are there you get the hard sell for a new vehicle at the full retail price.
This is exactly that. They are going to get a frenzy of people wanting the device and then will open the floodgates and then ask full price.
It is noticeable that this company came out of OPPO who sell an almost identical smartphone and charges almost $700 for that phone (sure there are slight differences in specification -- a slightly different size body dimensions -- but essentially it is almost identical in specification) and I am sure that this is a marketing campaign by OPPO to will 'merge' and the OnePlus One device's price will be priced at a similar price.
When something is too good to be true it usually is. There will be those lucky ones that got the cut price phone but eventually that price won't be available.
It's just a ploy to get a name for the company.
Biting the hand that feeds IT
