> I wonder how the TCO for a used SP and a used Mainframe would stack up
> against 6000 PC's.

The TCO of a mainframe is going to be less than that of 6000 PCs.  It's
also going to do less work than 6000 PCs.  Replacing 6000 high-end PC-based
servers that are all processing heavy workloads is going to take quite
a few mainframes, probably over 60 (and I'm trying to give the mainframes
the benefit of the doubt here).

I'm not saying that there aren't valid applications for mainframes, or that
they aren't cost effective.  But replacing Google's server farm may not
be one of them.