Unless you can find a great deal on a used frame or steal parts from a bike you already have or a used parts bike or great deals on components - it's usually cheaper to just buy a whole bike new.

If you build it yourself, you have the pleasure of having exactly what you want at the level of componentry that you want, and probably you'll end up with a higher end bike all around... but you may not be saving money.

The wheels on a pre-built bike are probably cheaper wheels than what you'd get yourself. Probably the disc brakes'll be heavier than what you'd pick...