Sprint does not have an unlocked iPhone available. The iPhone set up with Sprint as the carrier will work in other countries but you will be charged a very expensive roaming fee by Sprint.
Apple will start selling an unlocked iPhone 4s in November, but it will not work for Sprint or Verizon in the US, only ATT. But you will be able to swap sim in other countries. Not sure if Pakistan is one of them.
The pricing on the unlocked iPhone 4S is supposed to start at $650.