Here's what I've come up with:
Two Opteron 244s: $406
MSI K8T Master 2-FAR dual socket 940 motherboard: $248
2 1GB ECC/registered DDR400: $240
250 GB Serial ATA hard disk: $105
DVD-ROM: $30 or so
Case, floppy, power supply (550W, I assume, will be needed), and so on: $100
If I substitute an Asus K8N-DL motherboard, the price goes up $26, to $1155. That's an approximation, and doesn't include little details like shipping, but it's a ballpark number. The Opteron 244, 1 GB DIMM, and 250 GB drive all represent the knee of the price/capability curve.
The difference in price between the Opteron and the consumer-grade Athlon 64 is not big at all when you back off from the absolute top of the line. Two CPUs are better than one faster one for this job because Hercules is heavily multithreaded, and quite good at keeping multiple CPUs busy. Two separate CPUs beat one dual-core CPU for bang for the buck right now; if I get hungry for more performance, I'll replace the processors later.
I dunno if I'll buy it right away, but at least a good fast box isn't out of reach.