Hard to say without concrete examples. Quick look at iBuyPower says that they're putting in rather underpowered graphics card compared to other hardware, noname PSU and cheapest possible motherboard. All of those combined will show as $1-200 dip in the pricetag. Also the pricechart in this thread is getting old and outdated. And if you're comparing iBuypower with other pre-built market computers, there's no monitor included either even though the pictures on their web site might make you believe otherwise.