Someone at RFD is saying he asked and it's Halo 5 for $19.99. Is there a "deeply discounted" game every day this week?
The person I spoke with seemed to think so (I guess they're alerted via store email and supposedly only Halo was listed in today's mail... BTW, here's the part where I clarify that he mentioned the price to some other customer before I talked to him, which is why I was unclear on the price -- I'm not that much of a jerk that I talk to a clerk and then half ignore them

I'd been planning to trade Halo and a couple of other things in their promo for No Man's Sky (3 for $70), but mysteriously it dropped off their list. This totally explains it. So I traded it to Best Buy and got the $20 I paid Microsoft for it.
got my cheap Gears. I wish games would go on sale this cheap all the time
I couldn't do it in the end. They had a few copies left at the store I visited, but I've already got the whole series digitally on 360, and a backlog that I shouldn't add to (because I'd feel compelled to get to it before Gears 4).