The real question is how much they cost for 256 GB, since this thread has to do with the non-existence of a 512 GB upgrade *over* the 256 GB version for under $600. Also, are you referring to general market prices or what Apple pays?
Probably about a $400 retail difference between the 256GB and 512GB version. Apple always charges at least retail price for these things, without taking into account the value of the part you're upgrading from.
I don't think it's an unfair price for the SSD, especially since it's a custom built version to fit in the slim case. They can't just buy ready-made SSDs because they wouldn't fit in such a cramped case.
On the Macbook Air you go from 128GB to 256GB for $300, without a processor upgrade. $500 for 256GB to 512GB plus a CPU bump seems pretty reasonable to me. You'll probably pay at least that much for a third party SSD upgrade in the future.