I think one of the big selling points for going green in the data center is the savings that can be had. With the right solutions, a data center can go green for basically free when you factor in the amount of money that can be saved.
To help visualize the savings, here is a simple efficiency savings calculator:
http://www.42u.com/efficiency/energy...calculator.htm
Of course going green is important, but the way to make that happen is by showing the amount of money that can be saved. Especially when you consider that energy prices are going up faster and faster.