Train tickets can not be purchased online. May be only Pak Business Express is offering such service. Check their website.
PBE Business: 5000 one way (dinner, breakfast, wifi, tv included)
PBE Budget: 4000 (Same as Business but without dinner, breakfast, wifi & tv)
Karakoram Business: 4420 one way (Same as PBE Budget but with newer coaches)
Night coach express (AC Sleeper): 6000 one way (minimum 2 persons as its cabin for 2 or 4 ppl)