According to Canon's sensor white paper, a FF sensor costs 20 times an APS-C sensor.
http://www.google.com/url?sa=t&rct=j...75097201,d.ZWU
This is due to several factors:
1. Area of sensor, so the number of sensors manufactured per 8-inch wafer. This alone means only 1/10th the number of FF sensors can be made per wafer as APS-C sensors.
2. For a given number of contaminant particles on a wafer, a larger percentage of FF sensors will fail - 10 widely space dust specs will damage 10 of the 200 APS-C sensors on a wafer - 190 good sensors remain (95% or a loss of only 5%). However those same 10 dust specs will damage 10 of the 20 FF sensors on a wafer, a loss of 50%.
2. FF sensors require triple the number of photo-lithography steps, meaning they're slower and more costly to make.
According to Canon the total of these factors results in a 20x price difference.
Might be true might have some half truths not sure but it makes sense in the most part.