A colleague of mine is looking for a single speed frame for commuting. Her total budget is 500 for a complete bike. To me it seems like its all cheap crappy steel frames in that price range but let me know if you have any suggestions.
In all seriousness, I'd suggest buying complete online if you can assemble it yourself and can find what you want. Trying to piece it together by buying the frame and all the parts separately will be *vastly* more expensive.
Not to take away from Dante's thunder, but these guys seem to offer a few more options in terms of colors and accessories, if those things are of importance. However, I have no direct personal experience with their product or customer service.