Like almost everywhere there are people who want to take ilegal profit from tourists. Is difficult to say a thing when it happens, but sometimes taxi drivers make routes more much long, and make you pay also more. If you can take a taxi with a serbian, problem solved. If not pay attention to the route and the accountant.
Beware of the "Wild" taxis at the train station that are waiting for you at the train station. They cost at least three times as much as the regular taxis that are associated with one of the recognised taxi companies in the city. (The tourist trap taxis generally don't have a name associated with them, and usually have a yellow sign). Then again, even if you do get ripped off, these wild taxis are still pretty cheap by Western Standards.