Assassination Experience Points-

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Character Classes - - - DMG

An assassin receives 100 x.p./level of the character assassinated
    minus or plus 50 x.p. for every level the assassin is greater or lesser than his or her victim.
This is modified by multipliers for the degree of difficulty of the
mission -- SIMPLE (X 1/2), DIFFICULT (X 1), or EXTRAORDINARY (X 1 1/2).
The explanations for difficulty given under SPYING [are] used as guidelines here.
The experience given above is added to the regular experience earned for killing the victim, as if he or she were a monster. Experience is also given for the fee the assassin is paid.

Therefore, if an 8th level assassin snuck up on and surprised a 10th level magic-user in the dungeon and successfully assassinated him,
the assassin would receive 1,000 x.p. plus another 100 x.p. since the magic-user was 2 levels higher than he.
However, since it was a simple mission, the total 1100 x.p. would be multiplied by 1/2, giving 550 points.
This is added to the 2400 x.p. normally received for killing this magic-user, making a final total of 2950 x.p. earned,
exclusive of fees.