The weight of any material, including balsa, ply, and foam is .009259 X Density, per cubic inch.
For example, a 4 inch X 36 inch sheet (144 square inches) of 1/16th balsa, (.0625 inches thick) at 6 pound density, is:
(144 X .009259 X 6) X .0625 = .49 ounces
I have some (free) info on my site, if you want to predict other structural weights, at
http://homepage.mac.com/mikejames/rc...rediction.html