It's currently $4.99 at Amazon. From his announcement:
Antimemetics V1 is staying online at the SCP Foundation. I repeat: Antimemetics V1 will always be available to read for free on the web. Normally, when an amateur writer self-publishes on the web and then gains enough traction to be picked up for a publishing contract, a clause of that contract calls for the original self-published version of their work to be taken down. Not in my case. My contracts specifically acknowledge the SCP Foundation copy of V1, and they do not call for me to take that copy down. (Which is not something I could do anyway, even if I wanted to.) My intention is to leave that copy essentially untouched, indefinitely. I love the SCP project and I'm proud to have my work be part of it. You're alright.
However, all the paid ebook and print editions of V1 will be removed from sale at the end of September 2024. I can't give a precise hour and minute when they will go offline, because this isn't something I can schedule in advance, but I plan to start pushing buttons around midnight UK time on 1 October 2024.
This is regrettable but unfortunately unavoidable. It should be fairly obvious why I can't continue to sell Antimemetics V1 at the same time as my publisher is selling Antimemetics V2.
If you've never tried the X-Files-ish series, start here.