We migrated 4 servers running Windows Oracle databases to one Linux server (Dell Poweredge with external Scsi). The advantage to us is Redhat AS Linux is able to address the 8+ Gig of RAM for Oracle vs only 2 Gig for Windows standard server. The Enterprise CPU licenses we freed from the Windows servers are now utilized in a data warehouse project. We have a massive increase in performance (in part due to hardware).
TCO and ROI may be hard to quantify. There was a good paper here showing a 38% improvement on identical hardware using linux: http://www.interealm.com/technotes/roby/pentmark.html but it is gone now. If you have to provide numeric data you may be forced to use the vendor stuff. Like I said we have a great improvement in performance, but no methodology to prove it.
I was able to convince the executives based on the savings for consolidation related to the data warehouse ($40k a CPU is substantial), less administration costs (we have linux/unix servers up without reboots for years, that is pretty convincing compared to windows), less critical security issues, and the fact that Oracle develops on Unix/Linx and ports to Windows. Gartner has some great stuff on cost of administration for windows vs. nix.
Even CIO and CEO marketing material is hawking linux solutions at this point so they should be familiar with it.
Hope this helps.