$250, tops...even then you'd end up with a better system, b/c I dont' think they sell 1.2ghz thunderbirds anymore. Some of the other parts are upgraded though (i.e. barton-ready motherboard...biostar m7ncd, radeon 9600pro, 160gb 8mb HD), so yeah...$250 tops.
Counting my original cost + cost of replacement parts (some stuff broke, hence upgrades), I've spent maybe $800-900 total on this box. Not too shabby methinks.
Markfw900, your list of computers is impressive...good lord :Q