You should refer to each dragon skill. It shows the exact amount of XP gained per use
As such, there is no maximum XP gained for the dragon for each battle. It's all the matter of rage and utilizing the high-XP gain skills.
Here's the XP at max skill level. You can use it as a referrence because the table is pretty much the same for the lower levels:
Crushing blow - 34 XP
Treasure Searcher - 17 XP
Ball of Lightning - 46 XP
Mana Accelerator - 29 XP
Mystic Egg - 33 XP
Stone Wall - 39 XP
Dragon Dive - 28 XP
Battle frenzy - 37 XP
Burning Lava - 40 XP