Chatter is included at no extra cost for all editions. Chatter is also available to non-Salesforce users for additional costs (speak to your AE).

The pricing depends on the licensing and licensing depends on what the Salesforce.org client wants, i.e., based on the business requirement and the number of users. The kind of Salesforce.org a client should go for, clearly depends on the businesses processes that the client wants to automate either currently or in the near / distant future.

Make sure you bargain before you purchase the license. Salesforce always has the best deals based on your organization's size.

Understand all the hidden fees. Push them about storage costs.

Chatter is quite expensive. Be sure you make the mandatory change management after buying this tool. You'll need it to make the users create added value with Chatter.

Free of charge