Not counting my computer, which is my digital source, I paid $400 for my GLite/DPS (used) and $400 for my Adcom GCD-750 (DAC and occasional CD transport.) My headphones (HD-650s) were $340 shipped in late 2004. My power conditioner (Monster HTS-5100 Mk II) was $420 delivered. I paid $150 for my headphone cables (Blue Dragon Mk IIs) and $50-ish for my interconnects (Kimber KC1s).
Using $400 as the base price, my investment ratio works out this way:
Cans: 0.85:1
Source: 1:1
Amp: 1:1
Cables (combined) 0.5:1
Power: 1.05:1
That's in order impact on the sound. Clearly, the HD-650s are a pretty good deal.
Conversely, the power condition is of somewhat questionable value. It was the last item I purchased, and what I find is that clean power lets all the other components synergize with each other, so its value is not as easy to quantify. I certainly don't regret it.
The only items I bought new were the 650s and the Dragons. I got the GLite/DPS, Adcom, and Kimbers used, mostly off the 'Bay. The Monster was technically NOS, I believe. There's nothing gee-whiz or exotic in my rig, but I'm very happy with the sound of it. For me, at least, I've found the sweet spot in the law of diminishing returns.
Now if you'll excuse me, I have to go back to talking myself out of a pair of TakeTs.