From what i know, there is a long term plan in the making for the PN. The reason to go for 8 submarines is that soon you'll be having 2 Agosta 70s retiring, meaning by the time first Chinese sub is delivered, you'll be down to just 3 Agosta 90s, which will be nearing their refits (first boat made in 1999). Submarines will take a long time to sign, assemble and then sea trial. So the Idea is that you will have a minimum of 4 boats, and then every year or so you add 1, for a total of 11 AIP boats.....out of which at anytime 8-9 would be available for Patrol. Now that is a decent submarine fleet going in 2020s.
Ships on the other hand can be procured at anytime. PN knows that it doesn't have any heavy duty frigates, and that OHP debacle really thew the spanners in the work. So unless they get 5 more OHPs and get them something like GENESIS upgrade in the next couple of years, i don't see the US route happening. So Ships too will come from China.
Now the quoted figures for Submarine deal is 4-5 billions, which comes to about 700 million USD per boat.....that is an outrageously high cost for a 2500 ton sub class....you can have Scorpene/Type 214 in that money. So i think the deal involves some ships as well, improved F-22ps or Type54. But since the source would be China, and they have some really credible offerings for ships, i think PN considers Submarines more of a headache to customize and assemble than ships. Submarine technology is a whole lot different engineering than Ships. So when they see fit, they can order the ships and fast track them. I think China would be generously financing the loans on low interest, because a sizeable navy in Pakistan's own hands assures them that IN is kept busy on that front as well as the fast growing Chinese navy.
But at the end of the day, no one can deny that 5000ton air-defence frigates are the need of the hour along with a potent land based aircraft strike force.