I have modeled out a casting mold that has both interior and exterior mold elements. What I want to do is figure out what the negative space volume is between them so I determine how much castable material I will need.
SketchUp can calculate the volume of a solid. You need to make a single solid that corresponds to the space that will get filled with the material.
So, open each component that borders the casting volume, select just the face(s) that are on the border, close the component and paste in place. When you have all of them copied out, you can select them and make a new component from them (it may help if you hide the other components using the outliner). You will probably have to reverse at least some of the faces so that their front sides face open air and their back sides face to where the casting material will go. Then add faces as needed to close the bottom of the mold.
If all goes well, the new component should show as a solid in Entity Info and its volume should be displayed there. If it doesn’t, you will need to track down the issues that prevent it from being a solid and correct them. Solid Inspector 2 extension can help with this.
I suspect @slbaumgartner is offering advice and method after examining the existing included model, in which all the walls are isolated individually, and also not precisely aligned in a way that can make a solid. As the model exists, piecing together individual pieces ( or exploding everything) and stitching together individual faces to make a negative space may be the only way.
In case the model is haotic (like this one) copying and pasting walls is also a wrong solution. In this case, the best solution is start new with right workflow.