Like what was said above, typically cards are traded via TE (Time Elixers), pots (Potions), or other 4*s/5*s.
A typical (non-rare, non-event) 4* at lvl1 will cost you about 2-3 TE depending on the artwork/stats/rarity/seller etc
5* prices are MUCH harder to predict. Some seriously rock, and some seriously suck. Some are rare as hell, some are more common than 4*s (it seems like).
For instance, a Sonia 5* will cost you 40-50TE because she is SO INCREDIBLY common. She is sexy, but her commonness brings her price down. A Werewolf unit will cost you 180-200 (sometimes more) TE. This is because not only is the Werewolf ridiculously sexy, but she currently has the best stats of any 5* out there.
6*s I don't even wanna talk about right now >.>
So it kinda goes like that, build up TE, by trying to buy TE through luna (doing it this way is hard, because finding people selling TE for Luna is RARE) or get TE through event rewards, selling max 2*s/max 3*s, etc
Then gradually build up to your first 4* then to your first 5*