Microsoft Online Services Cost Estimator : 2,5 $/Month per user for Office Communications Online

Use this tool from Microsoft to compare the cost of your own infrastructure with the cost of hosted services with the same number of users :

http://www.microsoft.com/online/estimator/default.aspx

Here is a sample list of prices with this tools  :

 It cost only 2,5 $ per month and per user for Office Communications Online !