The Acer and HP Windows Mixed Reality Headsets are now listed on the Microsoft Store for US$299 and US$329 respectively. The requirements listed are much higher than what was originally promised, but since these are targeted at developers, those might only be the specifications for development.
https://www.notebookcheck.net/Windows-Mixed-Reality-headsets-now-available-on-the-Microsoft-Store.239852.0.html