The bonus is dependant on the dungeon. Ifrit for example gives like 75k+75k bonus. Satasha gives 110k+110k bonues.
But unless it's a trial you're pretty much guaranteed a level and a half as the exp scales inversely with level. The higher the level you are the more exp you get.
Further the amount of experience you need to level is synced to the level you are synced to. So if you get synced to level 20, you only need the exp from level 20-21 to go up a level instead of 50-51 or whatever.