If you read through the threads here, not counting a monitor, most of the systems people are putting together are easily coming in at $500 or less.
The key for you to decide is whether you plan to game on it (requiring a more expensive video card and a decent sound card), save alot of mp3s or other files (requires a bigger hard drive), or overclock (often requiring special heatsinks and other cooling, and a decent power supply). A fancy case might cost more too.
You can get micro-ATX mobos with built-in video, sound, and even networking. Do you plan to get on the internet? If so, do you have broadband or dialup. That will make a difference as to whether you need built-in networking or a NIC or you just need a modem.
Hope this helps?
