This Mi Box S is technically outdated, so it may very well be that availability is no longer so great. Then the prices often go up. However, I would no longer recommend the Mi Box S, because there is a newer and technically better solution with the Google Chromecast with Google TV 4K.

Google Chromecast with Google TV 4K (Google Assistant)

As an alternative, the newer Mi Stick 4K would also work:

Xiaomi MI TV Stick 4K (Google Assistant)