I can't find price info on Beograd-Sofia connection (return)
Full return fare (bought in Serbia/Bulgaria) is 41,2 EUR
Will it be cheaper to buy direct return Beograd-Sofia or two return Beograd-Nis and Beograd-Sofia (the second one seems cheaper comparing ZS and jizdenka-50%(SET))
Well, it is simple math: full fare Nis-Sofia is 9 euro (return is twice), cheapest (=Putnicki train) full fare ZS Belgrade-Nis is 784 RSD (or extra 100 RSD for Brzi/Fast train). Plus, at least if to believe ZS website,
pri kupovini karte u unutrašnjem saobraćaju, ostvaruje popust od 30% na redovnu voznu cenu, which gives additional benefit to "break" tickets