I had one of the more expensive after-market lightning cables (say about 50% of the genuine cable cost) that worked for about a month then stopped. When it stopped I had errors on the phone about it not being supported and errors on the computer about a USB device malfunction. So today I was pressing lightly on it to see if somehow the contacts weren't getting good contact (both ends were clean, no lint, dust, grease, food, anything) and without any real pressure at all the little metal end just popped off. Turns out on this particular cable there's only about 1/16" of metal embedded in the plug and the only other thing holding it is an extremely thin circuit board. I'm now assuming that board was cracked, hence no valid connection.
I've owned a variety of cheap knock off cables over the years, the issue I usually have is the cable separating from the back of the port. This one had a very solid connection on that side, which is why I bought it.
I've also owned a half-dozen or so genuine apple cables and so far the only design flaw that I've found in them is that I inevitably lose them. I've never had a genuine cable fail. I currently have 3 of them, one of them is the original from the phone (iPhone 5 from when they first came out).
On the balance of things the cost is probably about the same... I probably get 6 times the life from a genuine cable vs a knock off that costs 1/6 as much, and if I were more careful about not losing them, it'd be a much better investment to go genuine.