CP stands for Community Portal. Meaning you can get that missing sticker item through and only a Community Portal event. And those events, usually happen....what? once a month? or once every two months?
I am right there with you. Im missing about 100+ stickers and it drop rates are extremely frustrating. That is why, at this point, coins really dont mean anything to me, its all about filling in those missing blocks from the sticker book. The Portal events where they say the pay out is a "Legendary item" - it is actually not true for high rank players. High rank players who meet the CP minimum quota, will only get coins. That is why, when the RED chest is labeled as "Special item" - I actually prefer that, than coins, rather than the bogus "Legendary item" prize.
At this point, I have given up analyzing on what the actual EA/PopCap metrics looks like. It is so random that there really is no science to it.
I just grind for my weekly 50 rainbow stars for the 3 item chest. Then do it again for the 30 rainbow star chest, ... then the 10 rainbow star chest.
Then, grind another 50 in preparation for the following week's chest.
Thats what? 140 rainbow stars.
For you and other high rank players, Its all about the grind for rainbow stars.