OK, you're right. It seems that small form factors (Mini) and all-in-ones (iMac) are generally classified as "desktops". Though workstations (Pros) are not.
By "desktop", I meant "towers, mini-towers, and those horizontal towers that used to prop up CRT monitors". Computers that you could stuff high-end (but not workstation) video cards, and quad core processors into.
It's only recently that the iMac used "desktop" parts, and not "laptop" ones, isn't it?
It's valid to discount the Mini because when you buy a Mini you are paying for several very significant features that are lacking from a typical desktop. If you want to make a fair comparison to the Mini, look at home-theater PCs that will be similarly small and quiet.
The mini, by virtue of using low-power laptop chips and other techniques to reduce its size and power consumption, will always have a higher cost than a budget tower with equivalent clock speeds but a 250W power supply built by the lowest bidder, loud fans, and other commodity parts.
If all you are looking for is a cheap tower machine built from commodity parts, any of Apple's offerings will require you to pay a significant amount for features that you don't want, because the Apple machine isn't meant to be a generic box or to compete with generic boxes. The Mac mini, iMac, and Mac Pro all target niches that are different from the low-budget, ultra-low-margin desktop market.
It's really no different from saying that the MacBook Pro is an overpriced desktop - of course it is, because it is more than just a desktop. Apple doesn't make ordinary desktops, much to the consternation of almost every hacker who wants an ordinary desktop (but not a workstation) with a good unix-like OS. If Apple did make a generic ATX form-factor box, nobody would take exception to the idea that the Mini, iMac, and Mac Pro are in different categories.
Huh? What about the Mac Mini, the iMac and the Mac Pro?