I am not sure if someone has posted something like that in the past, but I haven't read it so far, and I thought it might be useful to the rest of the modders.As we all know, Blizzard has hadicaped the amount of experience given if the monster's level is <5 or >5 levels from the player's level. The following table describes how much of the normal experience you get:
+-6=88%
+-7=68%
+-8=36%
+-9=15%
>=+-10=5%
That means that, if the monster gives 100 exp, and there is a 8 level difference between the player and the monster, the player will get 36 exp when kills it. if it had 200 exp, the player would get 72 etc.
The minimum amount given is always 5% of the normal exp.
One more thing, levels also affect the "to hit" and "get hit" propability. It is harder to hit a monster when it is higher level than the player, and easier to hit when it is lower level.
Have fun modding! 
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I just know that I know nothing - Socrates