Even at $606 that still leaves $374 for the case, motherboard, optical drive and operating system. I suppose, $60 for the case, $120 for the motherboard, $20 for the optical drive and $100 for the OS, that leaves around $75 as a builder's fee... Not that unreasonable,
If it's prebuilt from a big name like Dell, ASUS, IBuyPower, etc... then I still think it's over-priced, but if a friend is building it for you, it's not unreasonable. If this is something you've found on the equivalent of E-Bay or Craigslist, run away!
Edit: Yes, if your system comes with integrated Intel (or NVidia or AMD) graphics, you can still add a discrete graphics card.