At launch of the i7 platform, dual channel kits were about $150 for 2 2gb sticks and tri channel kits of 2gb x3 were about $200. Those were 1600mhz sticks too boot. You could get 1066 or 1333 for much cheaper.
It was a 25% increase for 33% more ram.
However, now the old dual channel ddr3 kits are taking a nose dive because they require 1.8+ volts to run and the i7 platform is spec'd for 1.65 and lower. They're trying to clear out the older higher voltage stuff. It also makes sense for them to charge more because they have to put forward the same performance at a significantly lower voltage.
Additionally, you have to consider brands. Corsair is almost always the most expensive in it's category. I was just shopping for some ram and OCZ and Corsair had sticks with identical specs, yet the Corsair was almost twice as much. Corsair had big black fins for heatsinks and that was about the only difference.