You can get pricing information in your local currency and order the required license from our on-line shop at 

Avangate

(If the above link to avangate does not work,
alternatively order the license from Share-it )
 

An Exchange GroupCalendar license is valid for one Exchange server with an unlimited number of users connected to that.
Whether there are 5, 50 or 500 mailboxes / calendars on the server.  There is no fee per user.

Your  license comes with 3 years of telephone / email helpdesk and software updates. All free of charge!
After 3 years the software will continue to work but you will no longer be able to install new versions.