What Gyrene said, plus some food for thought:
I don't know about warranty laws in your whereabouts, here in Finland the consumer is protected by law for at least two years for manufacturing or material defect concerning products that are supposed to last more than just mere months or so and cost a noticeable amount of money - meaning almost anything. Basically even a €20 dvd drive falls into that category. Even when there's a sticker on the bottom of a <brand> laptop saying the warranty is for one year only, they automatically extend it to two or even more if you threaten them with authorities. The same consumer protection covers the whole chain: Major brand manufacturers, small builders, shops, any kind of business. Check your local law.
If you buy the components separately for DIY, you might get extremely long warranty time for some and less for some other parts. Although most of the time they seem rock solid no matter what, sometimes a tiny spark of static electricity can fry a component. In that case it'd be your fault and out of warranty. If that should happen to a professional builder, they'd have to suffer the damage.
There's lots of reputable builders in the Internet, giving perhaps a base model which you can modify from a drop-down menu. They should know that those who do their shopping in the net know the price range quite well, which should keep their component prices on a decent level. Local builder/component shops have long known the same. I like the locals, you can always take a baseball bat with you if they try to pee you into the eye. OTOH a larger company has a bigger name to lose, so choosing a reliable builder always means some homework. What I'd like best would be a local subsidiary of a bigger company, getting volume discounts and taking care of their reputation, thus giving you both a decent price and good quality.
The question, how much is much, depends on whom you ask. I just did a quick calculation from a major local netshop/-builder for a mid class i5+radeon7770 rig and it looked as the price of the components would be very close to their ready built option. Compare, compare, compare, and when you think you've found something, put the compilation here for evaluation.
