"For simple information how to reach somebody, where to find him and how to approach him so I don't get killed, I suppose I could offer 100 gold.
...But if you were to make a deal with somebody in my place, so I can skip the haggle and get a ready stream of orders... and avoid the suspition of being a cop and trying to find info on your bosses... this would be worth much more.
How much... depends on the deal mostly. If I can get materials for a fifth of the base price and sell the ready product for ... lets say 60%... or any other combination which would net me 40%of a base price as profits... and I give you 500 for making the deal. If you can find a better opportunity I would pay more, or maybe you would just take the extra profits yourself..."

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This is based on the part where materials for poisons are valued at 1/6th of the price if they are easily accessible.