We do not have storage limits on mailboxes.  There are aadditional charges based on storage overage though.
Storage is aggregated at the organizational level.  If 10 users each get 500MB of storage, they share a total of 5,000MB of storage.  As long as the organization is not over their limit of total storage there are no additional fees.  Additional storage is billed after notifying the administrator of the overage, if they are still over their limits 14 days later.  Additional storage can also be purchased.