There are three mobile operators to choose from in Serbia - Telekom, Telenor and Vip. They offer similar prices, but it's always best to sign up for yearly contract, as you get additional discounts. In my opinion, Telekom is the most affordable one and my monthly budget is 750 RSD (less then 7 €) with 300 minutes, 300 messages and 500 MB of internet at high speed + unlimited mobile internet at lower speed after the 500 MB is spent. The full price of this package is 1400, but you get half the price if you sign up for one year contract. I hope this helps you.