IT World takes a look how much Windows Vista will cost for businesses:
I estimate each Vista user will cost your company between $3,250 and $5,000. That's each and every Vista user. Money will go to Microsoft for Vista and Office 2007, to hardware vendors for new PCs and components, and possibly a few bucks to Apple for those users jumping to a Mac. After all, if Apple's higher cost has been the factor keeping your company from trying a Mac, that factor just washed away.
Why $3,250-$5,000? Here's my calculation. Feel free to tell me what your company has budgeted, and whether you believe your own numbers.