About an hour out of town there is a wood mill that sells huge bundles of scrap wood for $5. They also are a source for free sawdust for the compost pile.
Originally I bought the wood with my neighbor as firewood. When I say a "Large" bundle I mean LARGE. When it is tightly and neatly stacked it measures three feet by three feet by four feet. Each of the pieces is in the shape of a 1/2" x 5" slat.
When designing the shed I came up with the great idea of utilizing the scrap wood slats as siding. It gives the shed a nice weathered barn board look about it. I've almost finished siding the south side of the shed and have only utilized a quarter of my stack of wood. I figure to side three sides of the shed it will cost about $40 including wood and nails.
Not bad on a tight building budget... score!