The world is full of them.....
(idiots)
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Because they research and develop a phone from scratch. They have an entire manufacturing process.
One Plus One have basically partnered with Find and rebadged the Find 7a. Thereby they have tiny development & manufacturing costs in comparison to traditional manufacturers...
Likewise they won't be spending $350 million on marketing the handset, with indications that they are likely to struggle to push 12,000 units out at launch. Hence the 'invite' system...
Compare that to likes of Samsung & Apple who will sell a million units in first week or so....
So that's why it's significantly cheaper, because R&D costs were low, because manufacturing costs are lower and because they are not spending millions on marketing and finally because they are not spending lots of cash getting the devices out across the world, into stores, into carriers... They are distributing from central hubs and via an online store only....
Take away all those expenses (logistic, R&D, marketing & manufacturing) and you can sell something cheaper....
The reality is however without spending more on marketing & logistics the chances of the device ever posing a threat to even the likes of HTC (who sell considerably less than Samsung & Apple) is very small.
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