I'd pay a premium for it even, which makes you wonder just how much money Oura must be making off the data of its users... (if they weren't, there'd be an incentive to sell the hardware)
You can get similar HW for $10-$15, and not the easiest hack but looks doable: https://www.youtube.com/watch?v=w90RVspTkt8
I'm surprised Oura hasn't pursued using the ring as an input method as the guy suggests in the video. It'd be super cool to like double-tap the ring then use it as a scroll wheel when running and listening to music/podcasts, interact with AI models, etc.
I have a miband I use with gadgetbridge. I'm reasonably happy with the app, and it has visibly improved over the last year (it also wins by default being opensource + the only option for keeping data private) but the watch is a bit bulky when sleeping or typing so I stopped wearing it.
I can't imagine $10 hardware will be particularly accurate, but cheap price + data control is enough to give me an excuse to play with one.
[1] specifically rings intended to be used by the QRing app - https://gadgetbridge.org/gadgets/wearables/colmi/