Maybe you should stick to Lego games.
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We have been getting 2 keys max. Most of the time my partner doesn't get a key or only gets one. I get 2 every time. It seems to be damage based, but we're not sure. Sometimes he kills way more than me, but I'll still get 2 and he gets 0. If you only do the objectives you will get 0 keys. We haven't been able to figure out what determines how many keys you get.
Theirs another mission, in the Madera Area, with a similar goal, or it might be norther Marin, by the Madera border. Essentially go in, activate 2 things, and done. usually awards me 5 Keycodes, now according to the patch, only emergency's are removing keycodes, so my guess is people will go to speed running The Motherlode, and 101 Explosions, I can consistently do both in 7 minuets with my clan-mates now.
that's 5 keys, good money, and if I run with my Officers, at times we have loot for clan, and group((each member)), and once I had self going as well. and Co-Op instances have higher drop rates, now add gun drop rates being increased post patch, my guess is a co-op instance can earn 5 keycodes, and nearly 200k in resources per run. Now, you can consistently go threw in under 7 minuets... well that's a lot of Keycodes in a short period.