The main reason that you can buy built wheels cheaper than building them yourself is economies of scale.
Also, Jobst Brandt says it's okay to build a wheel with used spokes, as long as the spokes are high-quailty. He doesn't give any qualifications here, either. (I was reading
The Bicycle Wheel last night before bed. Really.) It's on pgs 80-81 in the 3rd edition:
Stainless steel spokes are more durable and reliable than plated spokes, but their main advantage is that they remain clean and bright and permit retruing after long exposure to weather. Plated spokes often rust solid into the spoke nipples. They eventually break at the nipple, either in use or when attempting to turn the nipple. On the other hand, good stainless steel spokes can and should be reused when a rim wears out or is damaged.
This makes me feel better, a fair bit, because I trust Jobst's engineering-mind on something like this, and I just rebuilt a wheel with a new rim, but the same hub and spokes. Built it well, mind you, to where I didn't expect problems even before reading Jobst's bit.
My take is that the build-with-new-spokes-every-time came up becacuse
(1) spokes are usually the cheapest part of the job
(2) if the wheel was poorly built before, especially too loose (in all or some spokes), then spokes may be well along in fatiguing. But if the wheel was well-built before, or if the spokes were too highly tensioned so the rim failed by cracking, then things are probably okay.