So, in an interest of knowing how often I can expect what rewards, I have begun the painstaking process of going through missions and documenting every gain. After I get 500 (I may go to 1000) points for a mission with an uncommon, I may do the same with no "Special" rewards.
The goal? Find what mission is ideal to do that matches silver gain with card gain and costs to boost a rare to max. I suspect it will still be 2-4, I just want to do a granny study ("See, I knew it/Told you so").