ram1009 :
It's a slippery slope. Why not pay another 10% and get another 2% performance increase? Where do you stop? As you say, it's in the eye of the beholder. IMHO, anything beyond a Phenom II x 4 or an i5 is a urinating contest for those with more money than common sense.
Ultimately it depends on the builder and what he or she is after. If you want performance, then you have to pay for performance. Of course, there is such a thing called "diminishing return" which is what you are talking about. After a certain point, the extra bit of performance will cost you more.
For me, I look for good performance based on my personal expectations. As long as I can obtain that particular level of performance within my budget, then I have no problems paying for it. If the cost of that performance is too high, then I would need to scale back my expectations.
Additionally, I also prefer my PC to as quiet as possible so I usually spend extra money (maybe around $100 +/- $25) on quiet fans and huge heatsinks. But that's just me.