Sherburne County sells off PCs after 3? years.  $40-$100 depending on
what's in it.  Today they have 3 of these, at $50/each:

HP DC7600 CMT PC, 40GB SATA Harddrive, Pentium 4 3.0Ghz Processor, 1GB
Ram, IDE DVD-Combo Drive, WXP SP2, Purchased mid 2006.

They take the machine, restore the factory image to it, and sell it.
New ones go up as they cycle them out, usually in batches.

Here's their surplus page:

http://www.co.sherburne.mn.us/admin/surplus.php