expired CDKeys.com: Call of Duty Ghosts $6, COD Advanced Warfare $7, COD MW 3 $6, Metal Gear 5 $6.70 (PC, Steam)
EDIT: Some of these games are even cheaper now at Black Friday time, around $1 AU cheaper .
I was looking to add some of the older Call of Duty games to my account, and these were the best prices I could find for non-region locked versions (note that Activision games now aren't just activation locked, but also run locked). The prices on Steam for these old titles are crazy expensive, and never seem to be more than 50% off. CDKeys seems legit; just note that your codes are scanned from box copies that redeem on Steam, so you'll have to manually time in every digit rather than copy and pasting codes from non-grey market stores like Fanatical.
If you add yourself to a Facebook mailing list, you can use a 3% off code twice: https://www.facebook.com/cdkeyscom/app/393344207416996/
Prices in $AU:
Call of Duty Ghosts: $7.09 - https://www.cdkeys.com/call-of-duty-ghosts-pc-cd-key
Call of Duty Advanced Warfare: $7.99 - https://www.cdkeys.com/pc/games/call-of-duty-advanced-warfar...
Call of Duty: Modern Warfare 3: $7.09 - https://www.cdkeys.com/pc/games/call-of-duty-modern-warfare-...
Call of Duty: Black Ops: $8.89 - https://www.cdkeys.com/pc/games/call-of-duty-black-ops-pc
Call of Duty: Black Ops 2:$10.69 - https://www.cdkeys.com/pc/games/call-of-duty-black-ops-ii-2-...
COD Infinite Warfare, and Metal gear 5, are also dirt cheap, but appear to be Europe only codes.