Several aspects of being a mercenary can be covered with disadvantages such as Bad Reputation: Mercanary and Greed. Mercenaries are usually reluctant to do anything unless it pays and as such when there is no profit in a fight they aren't entirely willing. To repressent this I have made a new "disadvantage," choose a combat advantage your character has, to which you attach the Conditional Advantage disadvantage, reduce the cost of that advantage to half. This advantage is not active for the first four rounds of combat or if the condition is met (i.e. for a mercenary with quick who is paid to fight someone rolls 2K1 dice for initiative. But if the same mercenary was to attack the same person without being paid for it the mercenary only rolls one initiative die.). A character may only take the Conditional Advantage disadvantage once. A Mercenary should have at least one of the above disadvantages.
Conditional Advantage could also have other triggers for non-mercenary characters. For example a Crane playboy duelist who only has Balance when dueling in the place of a woman or a Lion samurai who has Combat Reflexes when facing a Crane.
Front line Grunt: 2 Koku per battle.
Yojimbo: 15 -20 Koku per year based on skill.
Yoriki: 15 Koku per year.
Bounty hunting: See Way of the Minor Clans page 105, oh and watch out for Wasps, er, Tsuruchi.
Assassinations: 20 Koku times the glory rank of the target.
Kidnapping: 5 Koku times the Glory rank of the target.
Notes: Increase all prices by 10% for Mantis Mercenaries as they have a certain reputation. All prices listed here are more or less guidelines, take into consideration distance traveled for the job, amount of resistance expected, and of course weird specifications (like kill this guy and put his head in his lord's room).
I think that's it. So remember students if a ronin asks you for 8 koku to fight for you see if he's good first.