We built three 12-foot shelves to span the entire width of a wall in our play room. Each shelf cost approximately $30, mainly because we had to purchase a higher grade wood to get 12-foot pieces that were reasonably straight. I haven't filled the screw holes with wood filler yet.
Because these are child-height and we have a little climber on our hands, we screwed the shelves into every stud along the wall, and used more screws throughout than we would have for simple display shelves.
This was my first project and I did most of the work myself, with my husband's assistance when extra hands were necessary. It turned out so well I plan on tackling a few more Ana White projects! :)