How exactly do you figure that? A C2D 3Ghz desktop with 4GB of RAM should sell for about $500. The iMac with these specs sells for $1700. So the display in the iMac is hardly "free".
I was comparing Apples to Apples
Consider that a 2.8GHz i7 iMac with 4GB of RAM and 1TB drive is $2200 while a 2.66GHz Mac Pro with 3GB of RAM and 640GB drive is $2500. So not only are you getting a slightly faster computer with more memory and storage, for less, but you are getting a 27" LED 2560 display to boot!