They resemble real-life dollar bills, but are in bubble form.
Role in series
In this episode, Squidward gives Mr. KrabsBubble Buddy's payment for the meal he had, which was a $100 bubble dollar bill. Squidward adds that Bubble Buddy also gave them a tip and shows Mr. Krabs a pile of bubble coins.
Cover of SpongeBob Comics #10 and Criminal Cover Pin-Up!
In these, the Bubble Dollars are shown leaving safe SpongeBob. There are also a few inside SpongeBob.