as per Vallis:


I invision an elaborate table that calculates the average casting cost of non-land cards -- something like this:

.
Ave. Converted Casting Cost || White Mana | Green Mana | Blue Mana | Red Mana | Black Mana
--------------------------------------------------------------------------------------------------------------------------------
Creatures
Enchantments
Instants
Sorceries
Artificats
Other

The headers themselves would contain values, as would the overlap. The resulting table could be composed of boxes spaced as follows, with the row and column headers listed above being the top row and left-most column. The top-left box represents the overall average conveted mana cost of all non-land cards in the entire deck, regardless of color:


O O O O O O

O O O O O O
O O O O O O
O O O O O O
O O O O O O
O O O O O O
O O O O O O

The only question that remains is, do the color sections represent the average converted cost of white cards (for example), or just the white portion of their casting cost. I tend to think that the later is more useful.