A Chest is an object in the game; they cost gold but drop an Item when opened. All chests have an initial cost, and the price of a chest increases at the beginning of every new map. This increase appears to scale with time.
There are 3 types of chests: Small Chests, Large Chests, and Golden Chests.
Small Chest[]
Small Chests usually drop Common items, but can occasionally drop Uncommon and even Rare items. They cost $25 on the first level no matter what difficulty you're playing on.
Large Chest[]
Large Chests are guaranteed to drop Uncommon and Rare items. They cost twice as much as Small Chests. They cost $50 on the first level no matter what difficulty you're playing on.
Golden Chest[]
Golden Chests can be found on Risk of Rain (Level), Temple of the Elders, and looping; they are guaranteed to drop a Rare item.