I'd say, probably $300/year, more or less depending if it's a "major platform upgrade" year, or just an interim/peripheral-upgrade year.
My current AMD rig, cost ~$145 for the MSI KT4V-L mobo and retail XP1800+ CPU, 256MB PC2700 was a present, but it cost ~$70 or so I think, HDs I had before, and pick up occasionally on sale when I'm out of room, monitor/keyboard I had, new "moddable" case with built-in blowhole was ~$40. A year later added 2 x 256MB PC3200 to the mix for ~$90, 4X DVD burner for $35, DVD discs... well, I assume that we're not counting software and blank media costs in this, just hardware upgrades, right? I probably spend $100/year on media too. I can't recall the last time (if ever) I bought a "complete" system. I've always done things piecemeal. Sometimes you save money, and sometimes you waste money, as the components that you've already purchased are devalued, while you wait for the "perfect" matching component and price to go with it to finish the build.
Recent additions to the collection include a full-tower Chieftec case ($45), 360W + 520W PSUs ($60), mobo and asst ($45), so it looks like 2005's computer hardware budget is half-spent already. The 2005/2006 timeframe will be my next scheduled major platform upgrade most likely. By then, PCIe perripherals, and chipsets with SATA-2 should be in full swing, and possibly consumer-level dual-core CPUs too. I'm going to try to get by and bypass the first-gen PCIe-capable systems and bypass SATA, and wait until things mature a bit more. OTOH, my PC has never been a "gaming rig" for me, I'm more of a game-console + arcade person. (Shock, horror, to those that live and die by their gaming PC rigs. 🙂 )