I would not use level at all. That's metagaming. The assassin's reputation or reliability would be a major factor if an independent operative. Within an organization, the assassin's rank within the organization would, along with the nature of the assignment, determine pay scale. As others have noted, the risk of both the assassination attempt itself and the retribution of the law (or of the victim's family/associates) are factors on the side of the victim. The price may also be higher or lower based on existing rivalries or cultural attitudes. (Half cost to kill a member of the guild of bounty hunters, hated by the assassins. Double costs to kill a woman.) In some cases, the mission might simply be refused: the assassin might be unwilling to kill children, or a figure might be sufficiently important that it could cause general instability if he were killed, something not desired by the guild. There are also client-based modifications to cost: long-time associates and repeat customers may get a discount, members of disliked or distrusted groups are likely to be charged more.