I was reading the development blog about Negotiation, and feel it could go one step further, which is to add other factors, like Company’s renown, relationship with town/faction, and difficulty/conditions in as bargaining chips.
Asking for more money will usually lower the view of how settlement sees your company (no one like a greedy contractor). You may also get special conditions that you need to follow if you wanted more crowns (like a Patrol mission that need to be finish in 3 days, or a defending battle that need to be finish in X turn, or need to kill a certain leader (and not allow him to escape) in a fight, etc).