I believe I payed $650 for my original rig. Since then, I have added $40 in RAM, $160 in hard drives, $20 for a wireless card, $20 for a video card, and about $50 in other expenses (cables, a RAID card, a webcam etc).
I've gotten a lot of my hardware for free. I use old 17" CRT monitors, kindly "donated" to me by friends and picked up by the side of the road. Someone gave me a SATA CD burner, and I have numerous IDE hard drives laying around.