Pakistan is not a rich country and it can not afford at this time to change its assault rifle completely to a new caliber of 5.56 specially when doubts about the effectiveness of 5.56 have been raised due to the recent conflicts, a few days back was reading somewhere that troops in Afghanistan sometime had to fire 5 direct hits to bring down a talib. So now when change in caliber is being spoken of, going to 5.56 would be not a wise decision.
If cost effectiveness has to be looked into with minimum effect on operational capability, then the HK-417 is the best option. Reason being it has many variants with different barrel lengths making it into a useful weapon for CQ combat, as an assault rifle and also as a sniper rifle. One weapon working into different roles. Best cost effectiveness would be achieved as the firing mechanism for all variants same, only barrel length difference.
Other reason, as we have 7.62*51mm as our standard assault rifle round, HK-417 uses the same, we don't have to change the round, will keep the same ammo, which has great stopping power and is a deadly round. No infrastructure changing to be done for ammo, huge cost saving.
Another cost saving reason would be that HK-417 can also accommodate the magazine of G-3, so all the magazine of G-3 won't be going into waste and if HK-417 is adopted, cost of manufacturing millions of magazines would be saved too.
So the only cost incurred would be getting the technology to make the rifle minus magazines, which is off course not a big issue as we can slowly start manufacturing the new translucent polymer magazines.
Cost saving in replacement of ammo, infrastructure for ammo manufacturing, magazines cost saving, no problem on the operational capability of the PA, assault rifle with great stopping power and effectiveness.
In my opinion excellent option with respect to cost and effectiveness.
Heckler & Koch - Group Website
Modern Firearms - HK417 assault rifle