At a flea market with a friend, I saw obviously fake Dooney & Bourke purses; my friend said she had a few and they were the real deal. The quality was so poor that they weren't even worth the asking price. Two days later I found a real one in a thrift store for $15, which was less than the fake.
A knock off is cheap because it's made cheaply. Just not worth it. I've been buying quite a few Korean skincare items from a legitimate site. Amazon sells some of the same items but buyers complain that the items are different from those bought elsewhere (labels, color, etc.). You get what you pay for.