Dark Sun: The Tyrant of Tyr
City-States mint Ceramic Pieces (cp) that can be exchanged for 1 pint of water drawn from a city-controlled well. These ceramic pieces are traded as the main form of currency in cities. Wages are generally paid in cp. 1 ceramic piece equals one gold piece (gp) in a normal D&D setting. These ceramic pieces are generally made with small bits of metal and marked psionically to prevent counterfeiting.
These ceramic pieces are perforated and can be broken into ten smaller pieces, known as bits. Bits are used as change for transactions involving ceramic pieces. One bit equals one silver piece (sp) in a normal D&D setting.
In merchant emporiums, travelers can exchange the coins of one city-state for those of another for a fee of 5 percent. In practice, few people bother swapping their currency, since most merchants accept coins of any city (although templars could crack down on this practice).
Outside of the city-states, Survival Days make an excellent form of currency to be traded. Each Survival Day is worth 5 cp, unless traded in an area that has readily available water. This may devalue a Survival Day.