Probably about the same, or just a little bit more. This is because the monitor that Apple uses in the iMac is an IPS display, which tend to cost an arm and a leg. If you were to get a normal, $100 monitor, a comparable Dell would cost a few hundred less.
You're also paying for the iMac form factor.
EDIT: I got $1500 for a Dell with a blu-ray drive, 2.1 speaker system, backlit keyboard, a better video card, and the same i5. For $150 more, you can get a touch-screen LCD. The Apple came out to $1700, without the above features.