The Windows Vista Business Value Tool is a Partner tool that can be used to help quantify your customer's costs and benefits with Windows Vista. Please note that this is a business value discussion tool and not meant to be a licensing configurator nor a quote generator.
Check it out here:
Basically you plug in things like Number of Size of Company, Desktops deployed, Office Locations, Versions of current Windows OS, How you manage the network and machines etc.. Once you turn the handle you'll get an estimated cost of deployment versus the savings that deploying Vista will bring.
Warning: The tool only allows estimates based on current deployments of at least 200.