With the advent of the U.S. 10 dollar bill, which bears the Roman numeral X, "sawbuck" became slang for the bill, as people associated the shape with the sawhorse. The slang term "buck" originated in the mid-19th century in reference to the dollar.
In the mid-1900s "sawbuck" became street slang, apparently originating in Chicago, for a 10-dollar bag of marijuana. Since 1985 or so, the term has referred to a 10-dollar "bag" (actual bag or any kind of package) of any street drug (heroin, crack cocaine, marijuana, etc.).
Occasionally, addicts use the term sawbuck to refer to 10 dollars, particularly when they intend to use the money to purchase drugs (e.g., "Borrow me a sawbuck so i can get my sick off").
"Sawbuck" is also the name of a Chicago-based media production company that focuses on documentary films on the street-level drug world.
"Let me borrow a sawbuck so I can get my sick off."
zJoe, "Nah, zJaw I can't do it, between the rock and the dice I be broke."