Hardware is one of my hobbies, and no matter how many times I build and rebuild, I never get tired of it. Give me a price ceailing, and a description of what you will be using the system for, and I can have something speced out within half an hour most times.
Dell and HP and the like use lower quality parts. You will get the same processor, and probably the same name brand video card, but other than that, you will get a locked down and hard to upgrade motherboard, low speced RAM, typically a lower end hard drive and DVD drive, and worst of all, a power supply which will never support any sort of upgrade. Not the mention the terrible cases, and non standard form factors some companies love to use.
To know that you are being ripped off for the cost of parts, all you need to do is look at the price it takes you to upgrade from one video card to another, then compare the difference with the difference in the cost of the cards should you buy them elsewhere.
As for my time, sure it's worth something, but just because my time is worth something should I hire someone else to mow my lawn? The point of my post was that for a decent computer on a budget, you build your own. If you don't, it won't be as good or as cheap as if you had put it together yourself.
