Newegg again offers the OEM Microsoft Windows 8.1 64-bit, model no. WN7-00615, for $99.99. Coupon code "EMCAKNX36" cuts it to $79.99. With free shipping, that's tied with our November mention as the lowest total price we've seen. (It's a current low by $2, although most stores charge closer to $95.) Deal ends February 4.
Note: It will be eligible for a free upgrade to Windows 10 upon release.