It is derived from a handwritten 'p(little s)', an abbreviation for 'peso' in old Spanish-American books. http://www.askoxford.com/asktheexperts/faq/aboutsymbols/dollarsign?view=uk (You can see what I mean by little s)

Oliver Pollock sailed the high seas at the age of twenty-three, and settled in Carlisle, Pennsylvania. This young entrepreneur rapidly established himself as a wealthy and influential West Indies trader. Pollock moved his operation to Louisiana, where he amassed even more wealth as a trader, and as a plantation owner. His success enabled him to provide supplies to the Patriots’ cause in the Revolutionary War, and to maintain close contact and a degree of influence with Congress. Pollock’s success allowed him easily to purchase military supplies to support "the cause," as the Spanish Empire had an outpost in New Orleans, Louisiana. In his dealings with the Spaniards, Pollock used their currency, the peso. In true Spanish tradition, Pollock used an abbreviation for pesos, yet his penmanship made the abbreviation appear to be the transposition of the letters "p" and "s." Prior to 1775, the fledgling nations monetary system was in disarray, and needed to be revamped. By 1775, Congress decided to rectify the situation by backing all of its legal tender with the most commonly circulated coins that were, coincidentally, Spanish coins minted in the New World. Americans then began trading with "Spanish milled dollars," later termed "dollars," as Americans shed the "pounds" that were the vestiges of British rule. Congressman Robert Morris, to whom Pollock addressed his billing records, perpetuated the use of the dollar sign, and was the first high government official to give his blessing to the "s" with the two lines through it. The appearance of the dollar sign in print, in a 1797 book by Chauncey Lee, signified the acceptance of the dollar as a purely American symbol, much as is the bald eagle. And, no, the dollar sign formed by placing the letter "U" over the letter "S" is not an abbreviation for Uncle Sam, as some have suggested!

The dollar sign is ancient. It is a symbol of the "Goddess of the Earth", as proven in David Horowitz book "Thirty-three Candles". The seal is a serpent crushing two human thigh bones.

Why do our keyboards have a dollar sign with only one line ($)? One line means pesos, two means dollars.