The Nexus 5 is subsidized by Google so that it's sold nearly at cost or possibly even below it.
Google's business model here is that it gets people into the Play Store ecosystem, which is where Google really makes their money on Android.
OnePlus has no such business model, which is why they're limiting access to the device via their invite system.
One additional worry bead about this price point is that it means they're likely not funneling much money to Cyngn (Cyanogen, Inc) to support this device. For various reasons (mainly, the Cyngn guys being notoriously difficult to work with), Cyngn-backed devices get little to no community input on CyanogenMod builds.
As an example of what happens when you don't pay Cyngn much for a device, see the Oppo N1. Once Cyngn got what they wanted (experience with taking a device through the GMS certification process), they deallocated most engineering resources for the N1, which has since then received minimal level of support effort from Cyngn. The end result is stuff like location services being broken for 2 months straight in CM11 nightlies. Nearly everyone who bought the CM edition of the N1 switched to Omni, which is maintained on that particular device by three guys (disclaimer: I'm one of them) in their spare time. That's how badly Cyngn deprioritized the device - three guys in their spare time are investing more into suppporting the device than cyngn is. (Admittedly, we're making better use of our time too - see below.)
I expect users of the OnePlus One will see the same with the next Android version beyond 4.4 on the OnePlus - the team at Cyngn take the "no bug reports against nightlies" rule VERY seriously, and the results of that show in the quality of nightly builds that are maintained by them. (Many of the community-supported devices are supported by maintainers who have a thread on XDA, where they'll hear if a device has a major issue. The end result is that most people have a high expectation of quality even from nightlies due to the "community maintainer pays attention to what's going on" workaround, but you won't see that from Cyngn-backed devices.)