There's some questionable math in this piece comparing the cost of a Dell server running Windows and Exchange with an XServe.
Some of it has to do with his chosen configuration standards. I don't know which options he was clicking, but I was able to configure two similar servers, one Dell, one XServe and the Dell hardware was cheaper.
He also doesn't seem to realize that Windows Small Business Server 2003 exists and includes both Windows Server and Exchange Server.
I priced out two beefy servers and up to the 25 user mark, the Dell was $200 cheaper than the XServe. Over 30 users and the XServe begins seeing a cost advantage. Also, Small Business Server maxes out at 75 users so if your company is larger than that, you really will hit the numbers he presents.