You can use a Volume surface with the contours displaying to obtain the 0 depth contours. I would copy your current volume surface. Change the layer to a different layer to isolate the surface copy. Then explode the surface copy TWO times (the first explode gives you a block, the second polylines). Locate the 0 contours polylines and insure the polylines are closed. If not minor editing may be need to insure they are closed. You can erase all other polylines from the exploded surface.
Turn on the original volume surface layer.
Select Surfaces-Utilities-Bounded Volumes
You will be prompted to select a surface - select the volume surface
Then you select one of the closed polylines and the program will calculate the Cut/Fill of that region.
Cliff Patterson
Rocke & Associates
Fort Wayne, IN.
Civil 3d 2007, SP-3
Xeon 2.80 Ghz, 2.0 GB Ram