Moria/Angband (and all their spinoffs including -- debatably -- Diablo) base their systems on matching monster level to player level, to see what you earn, with rising XP-per-level requirements beyond that.
In Diablo, there are optimal levels of monster you can kill, within 5 levels of you in positive and negative. That looks something like this (apologies if this isn't accurate, Diablo fans):
XP Gain = monsterBaseXP * (5-|(characterLevel-monsterLevel)|) / 5
(where result cannot drop below 0)
Examples for characterLevel 16:
Monster A's level: 16. Gain 100% of monster base XP.
Monster B's level: 13. Gain 40% of monster base XP.
Monster C's level: 20. Gain 20% of monster base XP.
Monster D's level: 23. Gain 0% of monster base XP.
In Angband (the model I prefer), it's something like this:
XP Gain = monsterBaseXP * monsterLevel / characterLevel.
So if you killed a level 50 monster when you were level 5, you would gain 10x the XP that a level 50 character who killed that same monster would gain. I like this, it is high-risk, high-reward. I have lost thousands upon thousands of characters in Angband to this. But the one's who make it, well they are the legendary ones. I think I once hit level 29 instantly from level 1 ;) Likewise, a level 50 character killing a level 5 monster would gain 1/10th of the XP a level one character would gain for killing that monster. Further, because the base XPs also go up, this means killing low level monster can still net you some reward, but go too far down and becomes utterly pointless.
In both games, they additonally use multipliers on XP needed (on a rising scale), however, and the ratio between this and the optimal level monster XP given does get gradually steeper.
The simplest form of this idea is to do the following. Decide how many monsters of equal level to you, you must kill each level to grade (ignoring quest rewards etc.). Let's say 100 -- irrespective of your level. Say that if the player kills a monster of equal level to herself, she gains 1XP.
Using an Angband-like formula to reward increase & reduction according to monster level, your levelling table is linear:
...And so on. I find this to be the easiest jumping off point -- tweak as appropriate, include monster base XPs, level cutoffs, etc. to your heart's content.