As they have 30k users, they are obviously prepared to spend a decent amount of money on using your product therefore, I highly recommend that you follow the ro

As they have 30k users, they are obviously prepared to spend a decent amount of money on using your product therefore, I highly recommend that you follow the route of Google Apps, Yammer, Salesforce's Chatter (Chatter Plus) etc and charge on a per user basis as well as, charging an inital setup fee.