Just as a note I really don't care what chip you use, sense it's going to a good cause regardless, but sense the only thing the Duron needs is a MB I thought it might be a more viable solution...
Anyway, I looked up some prices (off pricewatch), and found a couple of boards that might be decent:
MSI w/VIA KT133 chipset:
Linky
-it's $25, but only supports 100/200MGz bus speed.
MSI w/VIA KT266 chipset:
Linky
-it's $27, supports 133/266MGz bus speed, but it's a microATX (only 3 PCI slots).
PS, Ray check PM