Seriously?! If so that makes me feel even better about my final choice of bike, as 'settling for' Tiagra was one of the things putting me off. So those huge price hikes are just based on weight and cosmetics? I'd read that lower-spec components are just as durable (sometimes more so) than higher end, but I thought the received wisdom was that 105 shifted better than Tiagra, Tiagra better than Sora etc.
Certainly when I test rode some bikes recently the Tiagra was noticeably smoother and quieter than the Sora. But I guess that could have been down to the way they were set up in the 2 different bike shops...
Smoother and quieter can easily be explained by set-up, cable quality and age, quantity of lubricant on the chain and the wear state of the chain. Al being equal, you won't hear a difference.
The difference comes in with the finish - DA for instance is incredibly beautifully finished - as well as durability and of course, weight. I doubt there is a lower end product that is more durable than its most expensive counterpart though. But then again, something like a derailer or a crank lasts so long that the issue is moot anyway. With things like shifters it is important if you do high-mileage in all conditions. Then durability is worth having. For instance, some of the Campag shifters have bearings instead of bushings like on the cheaper models.
The big price difference always comes with products at the leading edge of al product cycle. You pay for R&D, tooling etc. This trickles down and is thus cheaper in low-end products. If you compare to derailleurs - one say a Dura Ace and the other a Tiagra, you'll notice that the Dura ace one has almost no steel anywhere. Even the bolts are titanium. The cages are carbon or aluminium. On the Tiagra one, the mechanical design will be similar but the bolts will be steel, the cages steel, the spring will have a cheaper coating on, the graphics will be painted instead of laser etched, the jockey wheels will have bushings instead of ball bearings (or vice versa) etc etc. The difference in weight is quite phenomenal as well. It isn't all that obvious when you handle only the high-end product but if you handle both, the difference is startling.
But the beauty of cycling nowadays is that performance is not compromised on low-end components. They all work incredibly well. There is obviously a threshold below which a sport cyclist shouldn't delve. Some components with Shimano on them are really only meant for a few occasional rides directly after Christmas day and then meant to be scrapped. Nothing wrong with that. It is what it is.