Decided to revive this thread, get some diversity in our discussions.
As daring as it sounds, I think PAF might end up getting 2~4 A-330 MRTT instead of KC-135 and A-310 MRTT. I don't think EADS is keen on offering the A-310 MRTT program anymore, and as per article posted by Mani2020, has offered A330 MRTT to PAF. Now while A330 is more expensive than the KC-135, we need to keep a few things in mind.
Firstly, the KC-135 airframes have aged quite a bit, and we may not have access to many fly-worthy (10+ years worth of flight time) airframes. The USAF might choose to stretch them out as much as possible until replaced.
Secondly, a boom-only type for only a portion of the fleet might not be an economically feasible decision. There's no guarantee that PAF will fly a seizable number of F-16s until it actually has such a fleet in its hands. In addition, the vast majority of the fleet - i.e. hundreds of JF-17, FC-20 & Mirage ROSE-I - will refuel using hose & drogue. In fact, if Pakistan managed to get heavy-lift helicopters and make those IFR-capable, they too would use hose & drogue.
The A-330 MRTT can refuel both boom-type and hose & drogue aircraft, and since it is an MRTT (i.e. Multi-Role Tanker Transport) it also has a solid emergency-relief dimension. The MRTT can be transformed into an air hospital that can house dozens of individuals requiring urgent care...we can both transport patient from one end of the country to the other AND maintain strong medical support for them.
The cost of the A330 MRTT is indeed very high, there is no doubt about that. On the other hand, there are a lot of major air forces who are acquiring this system, including Saudi Arabia, UAE and its in the running for U.S's KC-X. So we should expect a reasonable number of international operators. Plus EADS might have learned from the loss of India (whose govt cancelled thinking A330 MRTT was overpriced), so they might be willing to relent on the cost or even offer Pakistan some offsets...this in turn might attract India back to the aircraft as well. And if U.S chooses EADS for the KC-X program, then reducing the cost of the A330 MRTT might be a more than reasonable proposition to net even more potential customers.
The other possibility might be KC-767s IF cheaper than A330 MRTT.