Co-developing is much more expensive and time consuming than buying ready to use products. In addition there is no guarantee of sucess. So at present looking at PAF economic situation it is good to buy these rather than co-developing it. Later in future perheps ..
At first look, You can see difficult face of development phases of your own systems but At the end, You will gain more than what you paid because Missile sector is the most strategical systems than any others in terms of technology, importance...etc
In additions, For Example, When you developed an IR seeker head to integrate on your air defence missiles, It is possible to use same/similar seeker heads for precision guided A-G missiles. Same for guidance/propulsion systems. All missile sector is tied with eachothers and uses generally similar technologies from air to naval systems so Once you catched somewhere of this technology, Rest is coming easily.
The most important side of developing your own missiles is that You develop the strategic systems of your own such as Detectors, RF, Guidance, Algorithms... which can easily be jammed by Enemy forces If They reach those critical informations of your system. The history has lots of such tragic incidents about missiles.
and When you developed your own, You type the algorithms in accodance with your own needs. I mean If USA designated 5m flying limit over the sea to hit enemy ships for Harpoon missiles international version (Most probably, Original versions have more deadly attacking capabilities), You can set up such critical key specifications of your missiles in accorance with your own requirements.
SAme for Radars, Even If the capability of foreign radars are able to search the targets with a range of 100km's, The software developed for export markets, gives you permission to increase the range scale up to 40-50km so Even If Radar saw the targets with a range from 100km, The software is waiting enemy targets to approach your strategic buildings-systems in order to give Warning/Tracking/Engagement.
The interesting side of all those, Even If you know Which probabilities you can meet When you ordered international versions of foreign missiles, You have to paid tonnes of money for those...