Do it yourself can be done for less money, and you can end up with a nice finished product. But here's what most people don't think about before they start a project like this from scratch.
First do they have the required skills to engineer the bumper and complete it's construction, and second, do they own, or have access to the required tools for a job like this.
This last one can be an expensive hurtle for many to overcome for a one off project. Having access to a full shop with say a metal lathe, milling machine, drill press, band saw and a Mig, or Tig welder sure makes this kind of fab work a heck of a alot easier.
That being said, if they don't have the skills, or tools for a job like this, then i whole heartily agree with you, spend the money and buy a prefabbed kit, or better yet a completed bumper. In the long run it'll be easier, and a much cheaper way for them to get the bumper they want.