Giant Chest!

The giant chest is the cheapest chest you can buy from the store. At arena 8, where I am currently, the chest costs 490 gems, at lower arena levels the chest will be cheaper. The reason the giant chest is more expensive at higher arena levels, is that it contains more cards then. This is true for all the different chests in Clash Royale.

You can also get the giant chest by winning battles. The chests you get from battle follows a specific drop cycle that you can read about here. You will get 4 giant chests for each 240 battles you win, so 1.6% of the chests you get from battle will be a giant chest.