As everyone is trying to get into silver 3 after the bronze reward nerfs, prices for cards have gone up dramatically.
In the past the best way to acquire power was by buying gold foil cards. This has changed however. Here is a comparison of how much CP you get per Dollar from different cards.
The cheapest legendary (500 CP) on the market will give you 27CP / $1
The cheapest gold foil rare (2k CP) on the market will give you 29,4CP / $1
The cheapest normal foil rare (20CP) will give you 33,33CP / $1
Cheapest gold foil epics and legendaries are at 30-33CP / $1, but the cost of a single copy is so high that you need quite a bit of money.
So the clear winner in terms of cheap collection power is the Exploding Rats.
TL:DR
If you need collection power, the cheapest you can get is in new rare reward cards.