A 2,8 GHz 64-bit quad-core CPU like the Phenom II X4 630 from AMD costs (here in Italy, for the END USER) less than 80 euros! Which should mean little more than 100 dollars. How much does ANY hi-performance (with hi-end meaning up to 2 GHz) multicore PPC processor cost? And how much will cost the motherboards to support them?
Trevor said in some interview the PA6T (aka "mystery processor") was 500 US$ alone.
But there are also ppcs that have prices that aren't that dramatically nuts. Unfortunately these chips are rather rarely fit into the desktop computing domain. These are mostly low end chips like 512x or chips that include things not needed for desktop computing and hence are expensive (PowerQUICC and QorIQ) or high end chips that are out of question again (the big ones by ibm) or those that aren't available on the free market (like the XBOX 360 cpu).
What's left is some models with a fairly just-not-totally-insane pricetag like the 86x0 or some QorIQs. But they cannot compete against x86 on the desktop market. And it is highly debateable to use ppc longer than reqired for desktop computing. Freescale's going to push it now a bit again and I think they have a remote chance (probably the very last chance for ppc in a broader market).
We'll see in about two years time whether they are successful or failed epicly (again).
For MorphOS I think in two years time it will be highnoon: More or less all ppc Macs supported by then. So either
a) stop it then at all,
b) move to other ISA or
c) (most unlikely) use new ppc gear based on QorIQ Altivec.
From current POV i'd suggest b), but am still dreaming a bit about c) and am fearing a).
But today MorphOS is fun as it is.