I don't believe that one goes out and decides to just put together a bike, unless one has a specific goal in mind. Since, in my experience, it is a lot less expensive to purchase a fully spec'd bike.
Most of my bike builds start off innocently enough...I find a bike frame that I really like for a price I could not resist. The build goes from there and usually take 4 to 6 months just hunting for parts at a price point that I like. I don't go for a specific budget ratio relative to the frame.