reference this against the things you buy and it'll help

1000 power could be from a single untamed legendary,
or 1000 from a couple untamed gold foils
(epics also have a high collection power)
there's a ratio between cost:CP and a balancing act between what you really want for playing and what will give the most collection power. Once you have power covered, you can start to ignore that a little more, but early on it's good to learn this detail to get an edge.