I recently paid around $65 for a 2" x 8" x 5' piece of rough quarter sawn Sitka Spruce. Considering how many soundboards I can mill up from this one piece, it is a bargain but requires a large bandsaw to re-saw the boards and a jointer and surface planer. Another thing to consider are veneers. There are places on the web that sell exotic wood veneers by the single piece. For a headstock or top cover for a less than appealing cigarbox (some are nicer than others), it would be fairly cheap. I also see various species of wood for sale on craigslist all the time.
As Mark said: check for used and recycled materials. In my area there are a few Restore outlets. These are run by Habitat for Humanity and are donation drop offs for building supplies. I recently got a bundle of 6 six foot long 3/4 in. white oak stair treads for $8! (I think it might have been mismarked.....)