It more likely comes from a handwritten 'ps', an abbreviation for 'peso' in old Spanish-American books.
You can also see http://www.shmoop.com/atlas-shrugged/dollar-signs-symbol.htm... which discuesses this.
"The dollar sign? [It stands] for a great deal. It stands on the vest of every fat, piglike figure in every cartoon, for the purpose of denoting a crook.... It stands – as the money of a free country – for achievement, for success, for ability, for man's creative power – and, precisely for these reasons, it is used as a brand of infamy....It stands for the United States."