Mailboxes can be bought in the following packages. These are separate from Exchange Cluster and these users will not be visible in your GAL of exchange. Also, you will have a separate interface for these users.
20 mailboxes – $9.95 per month
50 mailboxes – $24.95 per month
100 mailboxes – $49.95 per month
200 mailboxes – $99.95 per month