With the price normally around £399/$399, this discount of $100 brings the Rift S in line with its modern replacement: the Oculus Quest 2; and making it sell for $299.
This new development makes it one of the cheapest VR headset ever produced.
Though Rift S is a good, well-recommended entry-level PC VR headset, its production will be discontinued in favour of e newer Oculus Quest 2.
However, they both have the same price
The standalone Quest 2, offers a number of performance improvements over the Rift S and is still capable of playing PC VR games when connected via Oculus Link.