The sand dollar serves as a secondary form of currency in the SpongeBob SquarePants video game Revenge of the Flying Dutchman, the primary form being doubloons. They also appear in the episode "Sand Castles in the Sand."
It is a type of paper currency and is worth ten regular dollars, much like a ten dollar bill in the real world United States. No amount of doubloons can equal one sand dollar.
Role in series
Revenge of the Flying Dutchman
When the Krusty Krab is shut down for financial purposes, Mr. Krabs suggests that he and SpongeBob run a food delivery business along with the normal Krusty Krab. However, Squidward is still unemployed, so he sets up a garage sale. SpongeBob, seeing a beehive he thinks is an acorn, wants to purchase it and give it to Sandy as a gift. However, the buzzing nut is priced at 200 sand dollars.
So SpongeBob has to earn 200 sand dollars from his Krusty Krab food deliveries in Downtown Bikini Bottom. Later on, he has to collect 300 sand dollars at Goo Lagoon in order to purchase a magic bottle from Rusty Scupper and gain access to the pier. Luckily, an abundant supply of reappearing sand dollars are present in the Goo Lagoon area.
"Sand Castles in the Sand"
They are a form of currency Patrick uses to pay a structural designer.