I was wondering about the Snopes explanation of "buck" as money, too; at least some dictionaries give its origin as unknown. I'd heard a very different story, that some version of the 10-dollar bill had a big X on it and was therefore referred to as a "sawbuck", and the 20 as a "double sawbuck". But I haven't found any recent corroboration for that and it is probably just another folk etymology.