Money is a form of currency that is used to pay for an item or service. Mr. Krabs is known to be obsessed with it as seen in many episodes throughout the series, beginning with the pilot episode "Help Wanted."
Known units and amounts
The money in SpongeBob SquarePants is the U.S. currency, loosely supported by the fact that Bikini Bottom is part of the USA. Known amounts include the penny, nickel, dime, quarter, one-dollar, five-dollar, and twenty-dollar bill. The largest known unit is a one million dollar bill.
The city of Atlantis has its own currency, but it appears to be gold, gems, and other seemingly golden objects. It is also said that the Atlanteans were not obsessed with wealth, as opposed to one of their visitors, Mr. Krabs. Nothing is known about the worth of the currency.
As seen in the episode "Mr. Krabs Takes a Vacation," all of the money in Bikini Bottom is made at the Bikini Bottom Mint. There, the money goes through different processes in order to be made. There is even a step where old money is burned up and created into newer money.
Dollar bills are made from "very special sheets of paper" that are specially prepared to be the standard for money. Coins are made from sheets of metal pressed into coins. Money is also recycled by shredding money and then processing them into brand new cash.
In the episode "Mr. Krabs Takes a Vacation," Mr. Krabs and SpongeBob actually receive their own money with their faces on it (though Pearl ends up taking them).
In many episodes, Bikini Bottom's population is seen to be quite small. In the episode "Clams," Mr. Krabs earns his one-millionth dollar, indicating millions of bills have been spent in Bikini Bottom, despite its small population. Mr. Krabs is also seen to have a large quantity of money in his vault at the Krusty Krab in various episodes.
Sometimes, money (mostly coins) have the ability of speech and moving the face of the figure just as a gag. Examples include: