Are there additional fees for mixing editions in Salesforce?

In general, there is limited mixing of salesforce Editions. You cannot mix GE, PE, EE, or Performance Edition: (editions) http://www.salesforce.com/crm/editions-pricing-b.jsp However, once you move to Enterprise or Performance edition, there is the opportunity to have some users at those levels, and some users on a "platform" license, which gives them no access to leads, campaigns, oppties, cases, products, and other "CRM" data, but they still have access to accounts, contacts, activities, and custom objects (and report and dashboards of course).