That price is differentiated because of floor is quite normal: Actually the higher floor you are on, the better view you would have and, the more cost would be allocated to you for elevator service and sewage handling.
However, this shall (not should) be included in rent but not be extra. And this rent shall be advised to you before you enter the contract, but not come into being all of sudden.
When I want to sign the rental contract, I am told I must pay another 14$/month, because the apt is in 14th level, is it common? Tks in advance