According to rules and regulations, the rent you pay must be reasonable. This means that the amount you pay should be in accordance with that of similar apartments at housing companies. If you have signed a subletting contract, the rent cannot exceed the amount the person on the first-hand contract pay for the apartment.
If you rent an apartment from a private person who owns the apartment (a bostadsrätt), the amount you pay should normally be a bit lower than the amount you pay for an apartment owned by a company (a hyresrätt).
If you believe that the rent is not reasonable, you can always contact Hyresnämnden and ask them to investigate whether you are paying too much. If you are, the landlord might be required to repay you the excess amount.
The monthly rent can be estimated accordingly:
(The value of the property* 0,04)/12 + monthly fee to the housing association + operating costs)