I just built a new desktop over last month to this one and here is what I have.
MSI P45NE02 Motherboard
Core 2 Quad Q8400 2.66 GHz Processor
4GB (2 x 2GB) DDR2 800 Mhz RAM
MSI GeForce 210 512MB GPU
350 PSU
Ubuntu 9.10 64bit

Mobo & CPU: $150
RAM: $70
GPU: $25 After Rebate
PSU: Reused one I got for $25 two years ago
Case: Reused an old PII case I like.
HD: Reused 80GB Maxtor PATA drive I got for $40 a while back
DVD-RW Drive : Reused PATA drive I got 4 years ago.

Next month I will purchase a SATA HD for $50 or less.

But the total that came out of pocket for this build is: $245
I like to see what I can build on the low end when I build machines.