Long story short...
Random CE drops
Form Factor
1CE
3CE
5CE
10CE
Chests (1 in 30?)
Rare Monster Drop (1 in 50?)
I realize this defeats the purpose of the "pay" part to the free to play game, but because of the limited gain, it could be seen as an added bonus to dungeon crawling, enough to get you an extra level or a revive while adventuring. Of course, banking all gains would take an incredibly long time to craft 3* or 4* armor using only these means, but would open up possibilities for players.
Not very smart, since monsters drop crowns and crowns buy CE.
So in a sense, monsters already do drop CE.