Depends on the year, but sometimes close to $1000. I bought a RAID 5 setup for my second PC, and that was kind of expensive - RAID 5 card, new case, and 5 hard drives. Several months later, a customer's 180GXP IBM hard drive died, making that the second IBM drive I've experienced the death of; the other was a 120GXP that had a severe head crash after the spindle seized up. So I am now in the process of doing health checks on the Hitachi/IBM drives that I originally bought for the array; the array is now comprised of Seagate drives.
So that was a pretty decent bunch of expenses right there.
Overall though, my PCs have been pretty stable upgrade-wise since I moved to the nForce2/Athlon XP base.