Balsa wood can be found in a large range of density.
Contest balsa is much lighter in density than the average balsa wood. Contest Balsa is generally considered to be less than 8 pounds per cubic ft. in density.
However...
Just because a piece of wood falls into this catagory does not mean it is a superior quality. It still needs strength. If a piece is really spongy it likely is not usable where strength is needed.

Robert