inboxes.com is for sale at 25000$.
I would do this from business perspective. Find out what users are willing to pay for what service. Define the service (enterprises, smb or private), define t-shirt sizings and interfaces (esp for enterprise and SMB - LDAP, ADFS etc), how to do the billing, then talk to an infrastructure architect for a bill of material and what the whole thing will cost you in maintenance and operations.