you see the problem for Iran air force is you want to answer what threat ?
Iran Airforce currently is consisted of many different Airplane from different Origin , that make it a maintenance nightmare.
as its stated Iran Air-force always wanted to have two type of fighter planes , a heavy air superiority fighter and A Lighter easier to maintain one like F-16 fr every day work and bombing Run.
if you compare Su-30 with airplanes like Mig-35 or J-10c you see Su-30 cost 4 time more to fly .also you cn't consider it a light fighter . while Mig-35 and J-10c can easily replace Iran fleet of F-4, F-5 , J7 or Mirage F1 .
here you can buy Mig-35 as its on the market for sale , but I never heard of J-10c to be for offer and also it don't have system like trust vectoring for engine and it can engage less target than mig-35 at the same time and also does any body knew the range of Radar for J-10c , the one on Mig-35 can detect a 3m^2 target from 250 km away. that's why I say mig-35 is good option for replacing Iran lighter air crafts.
now when you talk of Su-30 ,you must consider it's supposed to replace f-14 not F-4 or F-5 and honestly can anybody really consider it an Upgrade over F-14 for that we need a newer modern alternative.