This paper provides the results from two total cost of ownership (TCO) based studies to make a case for migrating from mainframes to open system alternatives.
The first case study is from a manufacturing company that migrated its SAP implementation from a mainframe to an Itanium based Windows 2003 system. (not exactly an open source system!).
The second study is from a financial services company that moved several computationally intensive applications from its mainframe to Red Hat Linux base servers. (borderline open source system!)
The Total Cost of Ownership (TCO) framework for the analysis is the takeaway here - you will have to step through the vendor product promotion to get there but it is really not laid on that thick!