Slang for a ten dollar US bill. Similarly, a "double sawbuck" refers to a twenty dollar bill. Sawbuck is also slang for a ten year prison sentence.
The term originates from the fact that US bills were marked by their Roman numeral equivalent into the early 19'th century. The Roman "X" - representing the Arabic "10" - resembles the crossed legs of a sawhorse. Sawhorses were orginally called sawbucks. From logging implement to cash in three easy steps.
The habit of referring to bills by their Roman numeral is also evidenced by the use of the term "C-note" to refer to 100 dollar bills.
Sources
http://www.randomhouse.com
http://www.languagehat.com/