It is not reasonable that a primary tenant charges a secondary tenant a rent which means that he/she earns money by renting out the apartment. The secondary tenant should pay the same rent as the primary tenant, provided the rent is equivalent to the apartment's utility value. 

You should pay a rent corresponding to the area of the apartment you are renting. If you are renting one room in a three room apartment you shouldn't need to pay more than a third of the total rent.

If the apartment is already furnished the primary tenant/owner is allowed to take a supplement charge of around 10 % of the rent.

Do not pay a deposit before you have seen the apartment and a contract is written