Negotiation improves payout of the agent by 5% for every level. Example you get 100.000 ISK for completing a mission. Then you will get 105.000 ISK.

Is it worth training Negotiation to level 5?

Depends on how much missions you will do. Will Missioning be your only way to make ISK then it is worth it. Let's take the old example again. 100.000 ISK will go up to 125.000 ISK thats 25% more than you would normally do. So if you make 1.000.000 ISK then you will get 1.250.000 ISK.

Are there other skills helping me make me get more out of each missions?

Yes there are. Go to this detailed forum via this link: [[1]]