If we talk about quality and 6-sigma (which roughly means 1 ppm defects), the 1 ppm refers to test escapes, not test rejects.
Even if a part only costs \$1, if testing fails to catch a bad part and we accidentally send it to a customer, where it fails in the field, the eventual cost (in failure analysis, corrective actions, documentation, time spent communicating with the customer, etc) can be much, much more than \$1.
There is also a benefit to demanding 6-sigma (1 ppm defects) or better quality from your vendors.
If you buy a \$1 part, and assemble it into a \$100 (or \$1000) product, where it causes the whole product to fail, then again the cost of that defect is much higher than the cost of the faulty part itself.