I just iced 140 cupcakes this way last night and I used alot of icing! I can't tell you exactly how much, but I can tell you what I used to make it. First off let me just say I am a self proclaimed Sandra Lee of caking. I love the decorating part, but I'm not so fond of the baking and making icing part so I tend to take whatever shortcuts I can.
I used a bucket of the Wilton decorator icing you get at Wal-Mart, 2 boxes of the Wilton buttercream icing mix (made according to directions but using all butter since there's plenty of shortening in the decorator icing), and a batch of buttercream icing I found on allrecipes.com that called for 4 cups of powdered sugar, 1 cup butter, 1 teaspoon vanilla, and 2 tablespoons milk. This filled my 6qt mixer almost to the top!
All I have left after a little bit of sampling, piping a few centers on some blossoms for half of the cupcakes, and the 140 cupcakes themselves is enough icing to do maybe 4 more cupcakes! So be prepared to make a lot of icing!! Good luck!