Below is the microsoft link that details the various editions of SQL Server 2008 R2 and their pricing.
SQL Server 2008 R2 Pricing.
Posted on by Swetha Sankaran, Microsoft MVP - Office 365
Published by Swetha Sankaran, Microsoft MVP - Office 365
Microsoft MVP, Office Servers and Services. President, Women In SharePoint DC. Co-organizer, Federal SharePoint User Group Office 365 SharePoint SME View all posts by Swetha Sankaran, Microsoft MVP - Office 365