The whole point of branding is to build trust (which admittedly they can then mark up on).
If you choose to buy a knock-off product, you are explicitly opting out of that trust, and are on your own. Maybe the knock-off is 95% as good, maybe it’s 5%.
I’ve generally found knock-offs of expensive brands to be not worth it. They’re entirely preying on a particular kind of customer, the kind who thinks they can scam the system by getting a cheaper knock-off. Pro-tip: that’s how scammers get you, by making you believe you’re scamming them.
Just go for a different, cheaper brand, one that’s building their own value, not riding on the coattails of others.
There are exceptions, but they are few, and again: the knock-off brands are counting on you trying them out hoping you’ll hit the jack-pot. What’s the worse that can happen to them, you try and get a refund?? 
