> a premium price tag?

$0.80 in bulk. Around $1.25 for individual quantities, depending on who your reseller is.

AIUI, the CPU cores are a small fraction of the die area compared to all the peripherals on the chip.

Yes, four cores in the chip. And yes, there's additional muxing, but I think that adds a fairly small amount of chip area compared with the crossbar. In addition to the two core slots, there are a lot of peripherals contending for single cycle bus access.