With 3200kg the payload of the F-5 is ok.
A Su-25 can haul more but the point is:
The F-5 was the last US "3rd World" fighter. Something that was designed to be cheap, cheap to operate and cheap to handle. Providing nations that could otherwise go into Soviet sphere of influence with a cost effective weapon was still a US goal in the 60's. After that and after the situation got stabile, weapons became just Business.
The F-16 already got a engine so complex and expensive that it went far from the idea of a efficient weapon, it was designed for US needs.
Soviets never had a "3rd world" export fighter but building fighters was not commercial business for them. They always sought a cost effective weapon instead, communist ideology.
So yes, the Su-25 might be comparable in overall costs to the F-5 and haul a ton more payload, but the IRIAF likes the F-5 for it's cost effectiveness. Alone the fleet size, infrastructure and experience make it a more favorable platform for the task.
Russia won't sell Iran Su-25 for $11 million, but we can build a Kowsar for 16 million with SAR-GMTI radar, modern navigation system, SVP-24 equivalent and advanced targeting pod, which the 11 million internal price Su-25 of 80's vintage don't has. All that makes up 7,5 million, means the airframe itself with engines costs just 8,5 millions, take 3-4 million away for the two engines and you build a supersonic rated non-composite material fighter airframe for 5$ million and that is for limited numbers.
You won't get a Su-25 with equal avionics from Russia for 20 million (5m export profit and 10m built cost).
You can get a single engine L-159 for $10 million with similar avionics.
You can get a twin engine YAK-130 for $15 million.
But neither are supersonic and none carries more than the F-5.
