Chest
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.