As a counterexample, Kirkland Signature's extra virgin olive oil fares really well in independent lab tests, while still being low cost:

http://www.laweekly.com/restaurants/best-bargain-extra-virgi...

But you're right, brands are supposed to promise consistency, not quality. A McDonald's burger should be basically the same at any rest stop, that's one of the reasons they took off. Some brands don't even offer that though.