Apparently you CAN'T patent or copyright a scent or perfume you CAN copyright the label and packaging only.  This does not mean that the only difference between a knockoff and a real perfume is a cheaper price.  On the contrary designer perfumes are expensive because they use better quality materials and most commonly use a combination of ethanol, water, and other "secrets of trade" to dilute the fragrance.  Fakes use BUTANE.  Which by the way is HIGHLY flammable and if you have a keen nose, it faintly smells bad.  Also designer fragrances are made so that they last longer with less application, knockoffs use butane and it evaporates very quickly leaving a weird oily residue on you.  Since we sweat during the day our natural oils mix with the perfume and (usually) gives off a unique scent while still retaining the integrity of the fragrance.  Since knockoffs use less quality materials, the blend my not be a pleasant mixture and the likelihood of it retaining is integrity are slim.
 
CONCLUSION: You might save money with a knockoff BUT, since you must constantly reapply you might end up spending the same amount you would have spent on the REAL THING or possibly more.
 

BASIC INTRO

Concentration levels

 Perfume oil is necessarily diluted with a solvent because undiluted oils (natural or synthetic) contain high concentrations of volatile components that will likely result in allergic reactions and possibly injury when applied directly to skin or clothing.

 By far the most common solvent for perfume oil dilution is ethanol or a mixture of ethanol and water. Perfume oil can also be diluted by means of neutral-smelling lipids such as jojoba, fractionated coconut oil or wax. The concentration by percent/volume of perfume oil is as follows:

    * Perfume extract (Extrait): 20%-40% (IFRA: typical 25%) aromatic compounds

    * Eau de Parfum (EdP): 10-30% (typical ~15%) aromatic compounds

    * Eau de Toilette (EdT): 5-20% (typical ~10%) aromatic compounds

    * Eau de Cologne (EdC): 2-5% aromatic compounds

 As the percentage of aromatic compounds increases, so does the intensity and longevity of the scent created. Different perfumeries or perfume houses assign different amounts of oils to each of their perfumes. Therefore, although the oil concentration of a perfume in Eau de Parfum (EdP) dilution will necessarily be higher than the same perfume in Eau de Toilette (EdT) from within the same range, the actual amounts can vary between perfume houses. An EdT from one house may be stronger than an EdP from another.
 
Furthermore, some fragrances with the same product name but having a different concentration name may not only differ in their dilutions, but actually use different perfume oil mixtures altogether. For instance, in order to make the EdT version of a fragrance brighter and fresher than its EdP, the EdT oil may be "tweaked" to contain slightly more top notes or fewer base notes. In some cases, words such as "extrême", "intense" or "concentrée" appended to fragrance names might indicate completely different fragrances that relates only because of a similar perfume accord.

An example of this would be Chanel‘s Pour Monsieur and Pour Monsieur Concentrée.
 Eau de Cologne (EdC) is originally a specific fragrance and TM of a citrus nature and weak in concentration made in Cologne, Germany. However outside of Germany the term has become generic for a weakly concentrated perfume.