AutoCAD Civil 3D :: Map - Calculate Area-proportional (weighted) Average Out Of Metadata

I have an acad document connected to some .shp files, which contain polygonal shapes with some meta-tags.

I want to achieve the following: draw a polygonal shape and select all the tangent areas (I already know how to do so)take a property of those areas and calculate an area-proportional [or: area weighted] average value  (proportional to the partial area defined by the drawn polygonal shape). I tried to achieve this by splitting the objects and merging them again with defined splitting (area proportional) and merging (average) options. The problem is: splitting & merging doesn't work until I disconnect the data files (checking the objects out doesn't work) and even then I can't split the areas. And, more important: This doesn't create a weighted average and I don't even want to manipulate the objects! I just want to get one value out of all these steps.

I appended a screenshot of my file. E.g. I want to take the value 923.2 and multiply it with the corresponding area percentage it represents in the drawn shape. So the calculation would look something like 0.02*923.2 + 0.2*926 + 0.12*945,9 [...] until all the 100% are covered and I get one single representative value.

I use AutoCAD Civil 3D 2012 on an Intel Core i7, 12GB RAM, 64bit Win 7 Pro SP 1 PC.