Levels and cultivation are related in a simple way.
Elementary Profound:
1st stage: Level 1
2nd stage: Level 2
10th stage: Level 10
Nascent Profound
1st stage: Level 11
10th stage: Level 20
And so on.
Now the SP he would earn for killing someone would be the same as his level.
If you kill someone level 1, you gain 1SP as long as they are at the same level or higher.
But if the enemy is superior in level, he is multiplied by the number of levels that he exceeds and also multiplies by 10, for every ten levels higher (In ATG it would be for each great kingdom).
Example:
The MC is level 15 and kills someone at level 27.
Gets: 27 (enemy level) x 12 (levels he is surpassed by) x10 (there are only a ten)
So you would win: 3240 SP
Example 2:
Level 52 MC kill a level 74 enemy
Gets: 74x22x20
Win: 32560 SP
If you kill someone of the same level or lower you only get the same value as the level the enemy has.