What is the cheapest flight from Dubai to Pakistan and what are the options to book it ?

From Dubai you have quite a few options and the cheapest is Airblue which costs very close to PIA. Having PIA you get benefits of having more weight carry limit and they do not impose any limit on the number of items you can carry while Airblue allows only one item to be carried (other than hand carry) up to maximum of 32KG. They will charge you at funny rate if you exceed this limit. Going to Karachi you get more options that i am not aware of but Islamabad has a limited number of options. You can also expect more options for Lahore.

I am not aware of any terminal related issues, yes there are three terminals here. Terminal 1 is for international flights like PIA (mostly flag carrier airlines) and is big. Retina scan, visa issues can be a night mare but if all is taken care of, you can be on the road in 20 min. normally. Terminal 2 is small (like ISB airport) and handles alternate flights from any country e.g. Airblue, AeroAsia etc. Terminal 1 and 2 are on opposite sides of the runway and it costs you atleast 35 AED (min. 10 minutes) to travel in taxi. I cannot go without mentioning that traffic can be very bad at times so you have to be careful in defining your stay. Taxi meter starts at 20 AED if taken from Airport.

Terminal 3 is the new one and is reserved for Emirates Airlines which will be relatively expensive. I am not sure about the latest picture but it will not be very different. Booking options are better discussed with travel agents as connecting flights is critical at times.