A casket is an item commonly acquired from monsters. Both the monster's combat level and your game mode play a role in determining the odds of receiving a casket.
The small chest contains anywhere from 1 to 50,000 coins.
Not to be confused with the wilderness casket, obtainable when killing monsters in the wilderness.
Below is a table of the effective casket rates per game mode and enemy combat level. Rates are slightly higher when in the Wilderness.