To all people saying this is a bloons ripoff, just because 2 games have similar features doesn't mean one stole from the other. That's like saying Zelda stole from Hydlide.
Obtaining cards is either costly or time-consuming. Pick your poison. The lower ranking cards need easier access; ie make crystals easier to obtain. Tyrant gives a good amount of gold for free players, it only takes a few games to get enough for a pack. Some ratio like that would be better.