I think, that the rare cards are quite cheap, the commons are very cheap, the epics are ok, but they can be expensive, and the legendaries are always quite expensive. But I think, the Shadow Energy prices are really expensive, and the keys are expensive a little, too
In my opinion, it’s best to buy perks and keys(mostly Epic and Legendary ones). Perks can offer great benefit in the long run, and buying enough Epic and Legendary keys can give equipment if higher rarity than it is possible to obtain by buying Rare items in the shop.
However, buying Rare equipment can help out in the first few chapters of the game, since higher rarity stuff isn’t available.