I am guessing their F-16's are in a bad shape. Some of them have already reached 30 years of service and few of them greater than 30 years of service. Americans being shrewd businessmen wanted to cash in before those F-16's can be permanently retired. They have at the most another 8 years into their life. Who knows Lockheed by that time might stop F-16 spare part production all together especially the older models. Most of Paki F-16's are not advanced. It looks like Pakis are desperate to keep F-16's going for another 10 years before they can all be replaced by JF-17's and J-10's.Half a billion USD is a lot of money getting drained out of their tiny fore reserves of $8 bn.
They are financially handicapped for the foreseeable future when it comes to expensive acquisitions. The only option is to extend the life times of the existing weapons. Pakis will be getting squeezed more and more as the years go by when it comes to military acquisitions making them dependent solely on China who will milk them dry by talking sweet words of higher than himalayan friendship.