Does anyone have a butter cream chart?  One that gives estimates of how many cups of butter cream per cake or cupcake.

Look at this chart and it will tell you an approximate number of cups of frosting/icing per cake size.  This is hugely variable because everyone uses different techniques/quantities to frost cake or cupcakes. 

Using a 1M tip for rosettes on either cakes or cupcakes seems to use the most, so I'd suggest you make a practice cake and a practice dozen cupcakes and measure EVERYTHING and arrive at your own numbers.

Look for the column that says:  Approx. Cups to Ice and Decorate

http://www.wilton.com/cakes/making-cakes/baking-serving-guide.cfm