It's a few transistors on a chip. It would be far, far more expensive to design, test and integrate a second version of the chip without the IGP transistors. You'd need huge volumes of the second chip to keep the price from shooting up.
Intel has the technology to power off chip sections dynamically - so your assumption about power could be completely wrong.
Nehalems power gates allow one or more cores to be operating in an active state at a nominal voltage, while remaining idle cores can have power completely shut off to them - without resorting to multiple power planes, which would drive up motherboard costs and complexity.
http://www.anandtech.com/cpuchipsets/intel/showdoc.aspx?i=3382&p=12
It's also possible to disable the graphics core by cutting the power leads on the silicon - but then you'd never be able to use it. With the dynamic power, the IG could be used for a second display or for lower power use when you want to stretch battery life. With the news that the IG can be used as a GPGPU, Apple could even use it for OpenCL or video decoding/transcoding.