The item shop just has card packs. There are four packs for each set: one for each faction. Each pack has 16 cards in total, and when you buy a pack, you get one of those cards at random. Each pack costs 200 GP. So it's 200 GP for one random card from your choice faction and set. Currently, there are four available sets (called "Episodes") and one EX set.
You can also buy the starters (these also contain materials) from other factions for 4000 GP each.
There's also a beginner pack that costs 50 GP for one card from the faction of your choice drawn from a limited card pool. It's not in my shop page anymore though, so either I'm not a beginner anymore or it was a limited time thing only.
The current GP prices are 1 South Korean Won per GP (there are GP bonuses though for bigger purchases), which translates to about $3 for 3300 GP. CPC is totally legit.
GP isn't just used in the shop, however, you can also use GP in the card training menu (you upgrade your cards here to give them better stats), or the item hunt menu (autofind materials over time), or the alchemy menu (exchange materials for other materials). It costs 500 GP for something in the card training menu (no idea what it does, maybe it gives you higher training success rates?), 500 GP for a treasure map in the item hunt menu (allows you to autohunt for better materials), and 100 GP for a reset in the alchemy menu (you can only exchange 5 times daily; this resets the count). You can also buy a camp for 200 GP in a dungeon that makes it so that you don't go down a floor when you lose a match (but if you lose to the boss, you still go back to floor 1).
They're just nice bonuses to have, but aren't too great. I think I'd rather just buy tons of packs instead.
As a side note, I'm only 17 Red Ore (and like, one Stockings) away from my Crimson Witch Maid~
Second best card in the entire faction. I'm going to have to use like half my deck as ingredients though