You don't need to handpick criteria in order to make Apple products seem cheaper. All you really need to do is include some criteria other than clock speeds and memory capacities. Geeks have a tendency to ignore advantages that aren't trivially quantifiable, even when those advantages have real monetary value to most consumers.
When you look at Apple's product line, you find that many products have no true head-on competitors. Most obvious are the iPod Touch, the iMac, and the Mac Mini. Those are products that clearly have a large market and large margins for Apple, so you would expect there to be some competitors trying to undercut Apple while matching them for features and capabilities. Instead, what you see are companies that try to undercut Apple by offering products that have significant disadvantages, such as ultra-small form factor PCs that only offer Atom processors and crappy Intel graphics, while still being bigger than the Mac Mini, or slightly larger boxes that are as fast or faster than an iMac, but when you add in the price of a good monitor, it ends up being several hundred dollars more expensive than the iMac, while lacking the convenience and not really offering much more in the way of upgradeability.
The only reasonable way to explain this is that all would-be Apple competitors lack either the engineering talent or the scale necessary to compete head-on with Apple's offerings. But if that's the case, then the "Apple tax" is no longer arbitrary - it is supported at least in part by very narrow and apparently natural monopolies Apple has in some niches.