Well, I'm sure Dell, Gateway, or Compaq would probably have *something* in or very close to that price range (at least after holiday rebates). If you don't want to build it yourself, start there. No smaller reseller would probably be able to hit that price point, if only because they generally use components that aren't total crap and they have to make a profit.
Do you have *any* specifics? Does it need a real video card? More than 20GB or so of HD? Minimum clockspeed?
For a really barebones, mininum system, I'd go with something like:
The cheapest AMD motherboard you can find with onboard sound, video, and LAN that isn't made by ECS.

Shuttle has some good prices. $75-100
A 300W power supply, maybe a "regular" Fortron? Don't buy totally off-brand power supplies (even if they're rated for "400" or more Watts), as they have an annoying tendency to break down and/or not work if you add more components or a better CPU later. $30, tops.
A Duron 1.4 Ghz (or whatever the slowest one you can find is). $50.
128MB of PC2100 DDR RAM. $50.
The cheapest 5400RPM HD you can find. $50.
A cheap CDROM drive. $20.
A cheap ATX mid-tower case. $50.
That would be $350, and I'm sure you can do better on some of those prices.