I put the following together just for a quick comparison.
Apex Mini-ITX Case MI-008, w/ 250W Power Supply - $44.99
Toshiba 2TB 7200RPM - $99.99
GIGABYTE GA-H77N-WIFI - $102.99
Celeron G540 - $44.99
Patriot 4GB DDR3 - $18.99
Asus Blu-ray Drive - $53.99
- Discount Code PREFERRED3 for 3% off order total
Tax: None
Shipping: Free
Total: $354.96
If we wanted to go full-on penny pinch mode, we could save ~$5 on the blu-ray drive, and significant amounts on the motherboard and HDD. For comparison,
here is how the Celeron stacks-up against the Atom 525. Something to note as far as OS costs go, there was at one time a loophole where MS would allow you to install upgrade versions of OSes as a full version and it was completely legitimate (was a bit of a backdoor in case the upgrade borked your system and you needed a fresh install). Not sure if they still allow this, but if they do, you could potentially get Windows 7 Home Premium for ~$50.
Edit: Forgot to mention the goal of the build I put together was at minimum to match every feature of the Asus linked earlier and in pretty much every case exceed it. Hence the extra spend on Motherboard with Wifi built-in and Blu-ray drive.