The rule of thumb with diy is that the first time you do something it costs more than the off the shelf.
Another thing to consider is that those cheap noisy power supplies are also usually inefficient, so a side benefit of an off the shelf solution is you save power.
Seasonic and Corsair are two brands I am aware of that make very quiet power supplies. I have a Corsair 450w in my htpc and I can't hear the power supply at all, even when I am standing next to it. If nothing but the pc is on, I can just barely hear the stock CPU fan from 8 feet away.