First, I don't know of any specific morale rules for NPC's/monsters, but I would say based on modern army tactics, a unit has lost its effectiveness at 33% losses, and at 50% its a rout. So if the NPC's break and run, do the PC's get full XP for the encounter? Any specific rules?
You get the full XP for the encounter. D&D's XP is awarded for "challenges passed." Thus if you cause your enemy to retreat you have passed the challenge of the encounter and should get he full XP for the encounter.