Look at what's on the market now: Two processors with identical cores, made on the same production technology, most likely from the same portion of the same silicon wafer. One is labeled Q9650 and cost $550, the other is labeled QX9650 and cost $1050. The differences? The QX has it's mulitiplier unlucked upwards, and it cost $500 more.
You don't need unlocked multipliers to overclock, so makes an "Extreme" processor worth so much more money? Nada.