I thought I read a while ago the true build costs for each phone, and the iPhone was like $250 for Apple to produce, and the Galaxy S5 like $225, these are not exacts costs. And they retail off contract for like $650, making like $400 per phone. Let's say the iPhone 5S sold in one year like 80 million iPhone 5S's, they would have made like $32 billion ?
So let's say the OnePlus costs around $200 to build, and they sell them for $300 to $350, making $100 / $150 per phone. So selling 500,000 phones at $100 profit they could have made like $50 million possibly.
Slightly simplified accounting there.
Build costs are only part of an overall cost that also includes distribution, logistics, returns, support, sales, retail, marketing, staffing, and many more. Only then can you talk about margin and profit.
Whilst OPO undoubtedly has lower overall costs due to direct sales and minimal marketing, I doubt they're making anywhere near the margin you're suggesting.
However, as volumes increase and parts become cheaper, manufacturing issues lower, yield rates hit their stride, they should be maximizing any margin (or limiting losses) as time passes - as long as they continue selling.
They would probably have made more per unit on SwapStyle cases had those ever happened.