Well the usual suspect for low cost non-integrated is ECS K7S5A, takes SDRAM or DDR and any socket-A CPU. It comes in several flavors, with stereo or 4-channel sound, and with or w/o LAN.
If it's to be all-in-one, including VGA, 4-channel sound, LAN, and a modem card packaged, then PC-Chips M810L is your friend - or if you don't reuse SDRAM, then the new 841LU. Although it's a shared-memory VGA solution, it'll probably still be a lot better than old leftovers.
These are all SiS chipset boards btw. If you prever VIA chipset, then either go completely basic and use the older K7VZA v3 (KT133A chipset), this goes for around $50, or get the K7VTA3 v3.1 with the current KT333 chipset for around $80-90, depending on which version you get. There are baseline, w/ RAID, w/ RAID and USB 2.0.
regards, Peter