in that respect, they both do the same thing, I am with you, but here are some areas where I am willing spend money, esp when it comes to weight. For example, the 7800 crankset isn't particularly light and is also $425. The 105 by comparison is cheaper but 150g heavier. In this case, I am willing to shell 3 bills for a carbon set, simply because of the almost 300g savings. But in specific ref to the seatpost, I found a road post made from 3al/2.5v Ti that was sooooo light (I think 121g) that the $35 vs $109 debate become moot IMO. I am also peicing together this bike so when all is told and the reality that I spent $500+ more than I had to on various parts of the groupo or other parts, it doesn't sting as bad cause i did it peice by peice.
The only part of the whole equation i have been out of the loop on for the past decade are all the advances in wheel tech. I am not surprised how easy it is to spend a grand or two, or three, on wheels, but I am not that advanced and am also rather, well, lets see... I have my own gravitational field, so stronger is better, leaving out many of the higher priced options. No radial lacing of 12 carbon spokes, no funny patterns, just a basic wheel that hopefully won't cost an arm and a leg (which makes cycling difficult).