If you're going to build your own, I find that techreport.com often has good information. Their system guide breaks down the cost of multiple types of computers.
The one they put out one on Sep 7th ->
http://www.techreport.com/articles.x/19560
prices range from $540 to over 3 grand (not including keyboard, mouse, monitor, and software).
It's worthwhile to just look it over to get a general idea about things.