Apple has always gouged on RAM prices...the gap had narrowed significantly, but third-party vendors have dropped prices a lot recently and reopened the huge gap. RAM is basically a commodity, and prices fluctuate daily. But Apple's not really a RAM vendor and I guess it's simpler (and more profitable) for them to set an initial price for RAM and then not adjust it.
Buy from a trusted third-party vendor like OWC or DMS and install it yourself. It's about the easiest thing you can do on your computer.