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Lens cost tends to depend on both optical/build quality and the shear size of the lens. Lenses with 3" front elements (like the 300/4, 400/5.6, 70-200/2.8) are all around the same size and weight and cost between about $1 and $2K - with the older lenses, unsurprisingly, being toward the cheaper end of the range. As folks have suggested on any number of forums, if/when the 400/5.6 is updated to include IS it too will no doubt be a $2K+ lens.
Start increasing the front element size to 4-6", though, and it's a whole new ball game. Lenses weigh 2-4 times as much as the 3 inchers, with some of that weight being big expensive Fluorite and UD elements. Add to that while the 300/4, 400/5.6 and especially the 70-200/2.8 are very sharp lenses, the 300/2.8, 400/2.8, 500/4, 600/4 and 800/5.6 are simply the sharpest and best built lenses Canon produces. So current generation of big white superteles use 3-4 times as much glass as the 400/5.6, are 20 years newer designs, have IS and are built like main battle tanks. They're priced accordingly.
Like a lot of bird shooters I worked my way up slowly (200/2.8 + extenders, 400/5.6, 500/4.5) before I finally sucked it up and bought a 500/4IS. For all my whinging about price that was one lens that was entirely worth it. Just an obvious step up in IQ and usability compared all those previous attempts. I haven't tried a version II supertele yet and almost dread doing so. If the jump in performance is anything like what I found when I got the version I I may just have to suck it up again.
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