When answering the question, "Are knock-off perfumes just as good as designer perfumes?", it is important to remember just one thing. The only thing about knock-offs is the price. In all other ways, the knock-off is inferior.
As someone who once sold knock-off perfumes, I know something of them. And the main difference between an original, designer fragrances and the knock-off is the ingredients. Perfume makers spend tons of money to create a fragrance. They use ingredients found in nature. Flowers, plants and other natural ingredients that are expensive to purchase. And they use a lot of these searching for the mixtures that will give fragrances that people like. Then they spend more money on unique packaging and marketing. Add all these expenses up and it is easy to see why designer fragrances cost as much as they do.
Knock-off producers, on the other hand, use only enough of the true ingredients to get a fragrance that is similar to the real thing. Then they use filler ingredients, such as butane, to finish off the fragrance. While these formulas create similar scents, the addition of other ingredients make it an inferior product.
The main difference is the longevity of the scent once it is applied to the skin. Butane, and other ingredients used in knock-offs, evaporate at a much higher rate than the ingredients used by perfume makers. An easy experiment, should you want to test this claim yourself, would be to spray two women with the same fragrance. One with the designer original, and one with the knock-off. After a few hours, at most, it will be obvious which is the better product. And should give you some clue as to why the original costs more additionally. The fact is, the original lasts much longer so less has to be used in the course of a day in order to maintain the fragrance. That's worth a bit more money, don't you think?
The other main difference between originals and knock-offs is the way the fragrance will change as it is worn. Designer fragrances are created to last, and one of the by-products of its longevity is that over time, the fragrance will change. As body oils mix with the natural ingredients in the fragrance, as subtle shift in the scent will occur. It is not enough of a shift to change the fragrance completely, but a very subtle change that blends with the natural scent of the woman wearing it.
Knock-offs, however, since they do not last on the skin, tend to not do this. And, if you have a very developed sense of smell, you can actually catch the odor of the butane as it evaporates. Butane is a natural ingredient. However, it is distilled from petroleum products and is,therefore, the next best thing to man made. It does not have the ability to blend with the body's natural oils, and therefor, just evaporates and takes the scent of the fragrance with it as it does, leaving nothing but a faint oily residue on the skin where it was applied.
So, are knock-offs just as good as designer perfumes? The only real answer is...Hardly! The only good thing about them is their price. And that's only an illusion anyway.