Because upgrading isn't constant every year for me, I'll just list each year starting from the one I got my first computer. So on average, $308.33.
2000: $800: P3 750Mhz, 128MB SDRAM, Viewsonic A90, Keyboard + Mouse, Creative 52X CD-ROM, Case, Westen Digital 4GB, (absolutley horrible) 200 Watt Power Supply, Voodoo 3.
2001: $600: (Just so you know, the power supply killed my system so I had to get another) Athlon Thunderbird 1.4Ghz, Voodoo 3, Creative 52X CD-ROM, Western Digital 4GB.
2002: $200: Hercules Prophet 4000 64MB, Western Digital 40GB.
2003: $150: 300 Watt Powmax Power Supply, 512MB SDRAM.
2004: $100: ATi Radeon 9200 128MB, Optical Logitech Mouse.
2005: $0: Nothing so far.