Assassin's Creed Unity Xbox One - Digital Code $1.79 from Cdkeys.
This game was half this price (less than a dollar) only a few weeks ago (also from CDKeys) but that deal has expired, and this seems to be the cheapest price around for this game atm.
It's been selling for two to three dollars for a couple of years now, so chances are that most people have already got a copy, but if you still haven't got one, the game is well worth getting.
Battlefield 1 Revolution is $4.89 at CDKeys right now. Cheap but not the cheapest (G2A have it for around four dollars).
I'm not sure it's even worth the four bucks, but someone else might like it (not a terrible game, just not my cup of tea).