High end (relatively speaking) cpus have always been disproportionally expensive. In fact this is true of nearly all hardware. A 1 gig ddr1 stick for example is more expensive than a 2 gig ddr2-1066 stick, which in turn is more expensive than a 4 gig ddr3-1600 stick. Core2quads are very expensive compared to socket1156/1366/1155 hardware, even though theyre slower. Socket 754 3700+ and especially 4000+ cpus are more expensive tha some new, much more powerful quad core cpus, etc, etc. Simple fact is it's often more expensive to keep an old platform and expand that with the greatest options for that system. It has always been like this, and it always will. Amiga gear even moreso these days, due both to the retro/rare value, and also the fact they offer some things that the "replacement" "NG" systems don't. Sure those things arent really attractive to everyone, but to some they are.
I have to second runequesters sentiment here though too, in that no matter how fast or well priced an x86/ppc cpu is, it still does nothing to enhance my amiga. That said though there's no way Id spend more than $350ish on an classic amiga accelerator, especially a csppc/bppc. Theyre of minimal use to os3.x and too weak to be of much use with a "modern" os (os4.x/mos) (unless you want to do nothing other than tinker with the OS or use existing 68k software).