Fry's *usually* has a house-brand ("Great Quality") box for between 150 and 250 USD; if you can hold off til tomorrow, you can check their big circular in the Republic.

 

It's frequently a Sempron (probably the old AthlonXP type, not the Athlon64 type), like 256M of memory, and a CD reader.

 

What parts do you need specifically?  You might be able to do better getting like whatever CPU-mainboard pair they have on special for like 100-130 dollars, and just throwing half a gig of RAM on it (total cost about 150-180 dollars, and you can buy a QUALITY power supply, which you may not get with a cheap OEM box.)