It works both ways.  Put yourself  in your landlord’s shoes. 

Its a great question and does pose an ethical issue for your apartment complex, but is not illegal in any way.  You would think that it would be illegal for a landlord to charge different prices for like properties, however, its not.  Think about your apartment as a business, like say– a car lot.  If you were to buy new car for $20,000 then return a month or two later to find that exact same car selling for $15,000, would you expect the dealer to give you $5,000?  No, but it would make you mad wouldn’t it?   Rental properties are a business just like any other that you deal with on a daily basis.  Landlords can charge whatever they want for their units, unless you live in a rent control district. (area that regulates how much a landlord can charge for a unit– this only applies to highly dense areas such as New York or San Fransisco)

Rentals are regulated by contract law.  No one forced you to pay the amount you agreed to.  You did have a choice and most likely accepted the terms at the time of signing because it was comparable to other available units that you had also considered.