It was a perfectly fair question and I answered it.
It is also somewhat rhetorical.
Why spend $400 when you can get better sound, lower power consumption, and less cables? There is no reason to whatsoever. If all those things were true and you could pay $400 for the new mini rather than $400 for a PSU upgrade then that would be a no brainer.
The problem is that the new mini doesn't necessarily sound better than the 09 mini with or without upgrades. It might, it might not. The only facts of this equation are: one, that my power supply is $450; two, it did make a substantial positive difference to the 09 mini; and three, that the 2010 mini starts at $700.
So if someone asked me what would make a more worthwhile difference: upgrading the power supply on the 09 mini for $450 or buying an entirely new computer for $700 I would say that's a fairly simple decision. The mac mini supply can always be resold for nearly what you paid for it, it's much more difficult to get your money out of a used computer.
However, if you wish to argue that a $700 upgrade outperforms a $450 upgrade then I think that is perfectly fair. It certainly is a more expensive price tag and one would conceivably think it should perform at a higher level.