Materials + labor = cost to build.
If you're building this computer yourself on spare time, you'll get a great value out of building a mid-high to high-end computer. Because, your spare time is ostensibly "free".
If building computers for others as a business, now that's a hard sell. You can't compete against Dell for price w/ warranty. But, where you can leverage your custom build options is in the ability to upgrade with non-custom standard parts. IMHO, it's not worth it unless the customer is willing to pay for my time. I'm not cheap, so neither are my computers. But that's ok, I'm up-front about it. Want a budget machine? I say buy a Dell or HP (refurbished too if they're that cheap). I'm not being snarky or snide. Just being 100% honest up-front. Saves everyone the grief.