The difference in pricing tells you a lot about the devices themselves. The iPhone is expensive because Apple knows it's a popular device, it's a device of value to people. That's demand, demand means you raise your prices. The OnePlus is cheap because sales are much lower than iPhone, and you lower your prices if you have more supply than demand.
Economics.
Apple does a great job of marketing and making people feel that they need to upgrade to a new phone and that they need to be apart of the ecosystem. However, there are tons of people waking up and saying enough by switching to more economical products. I wouldn't own an iPhone if it wasn't on sale. $400 for a SE vs $185 is a huge difference. As of now, a higher end Android (LG G6) is more economical than a higher end iPhone (7+).
Apple also keeps supply lines more narrow so they can artificially increase demand. They also know 32gb to 128gb is a huge step up in addition to better hardware on the + line, so they can get people to buy a 128gb 7+, especially with exclusive color choices for 128gb or more with Red and Jet Black.
Tim Cook is great with returns, but not so great with innovations. What good is the extra "horse power" on the A10 if you can't do as much on the 7+ as you can on the S8/S8+ due to software and hardware differences? What good is the extra real estate if there isn't much to enhance multi-tasking? Stuff like that makes me reluctant to even consider any OLED iPhone because I know I am paying a premium for a brand name, even if the value doesn't exist to me. Apple could make the best phone in the world with iOS 11 and its OLED model for all we know, time will tell. However, based off past trends, I am not too confident in that nor the pricing. I would like to be pleasantly surprised and would highly consider trading in my S7 for a "free iPhone OLED model" on contract or even a cheaper version on contract for like $200 (installment plans are the new contract on my carrier, T-Mobile).