From the website it seems the ring is $75. So definitely cheaper

The Oura Ring costs $350 now and it has an optional $6/month subscription.

Also, rechargeable does not mean it is infinitely rechargeable. After around four years, the Oura's battery will not hold a charge long enough to be useful (record a night's sleep), so it is also disposable.

We are comparing apples and oranges though because the Index is not a smart ring.