The Mail for Exchange app is available free of cost in Nokia store. But the app requires a data connection to sync content between your phone and your Exchange server. So, to be able to use the app, you have to subscribe to a data plan from your operator.
If you do not have a flat-rate data plan, the cost is based on your data usage. For Mail for Exchange this is directly related to the amount of email traffic you have on your email server. This depends on how you use email, how often you use it, and the corporate server you are connected to. The app connects directly to the email server and syncs email based on the sync settings you have made.