I'm a little rusty on licensing options now, since it's a while since I had that particular responsibility in my current company. However, last time I made a comparison, the water was muddied quite a bit based on the whole area of development environments.
At that point, Oracle charged quite a bit per processor for any production installation, whilst SQL Server was far cheaper, but required licenses for every installation. Given any of our mainstream applications have a production, test, development and (usually) at least one sandpit environment, that means SQL Server had to be less than a quarter of the per processor price of Oracle to be fiscally competitive. And that, of course, was before we took into account the high-volume capabilities of different platforms (the gap's closing, but at that point Solaris provided a clear advantage, effectively ruling out SQL Server at the first hurdle).
Now, if MS moved to a Production Instances Only licensing model, that'd really be something to think about.
Semper in excretia, suus solum profundum variat