Lungworth

I think it would be more profitable and cheaper to just have a drinking party.

By the way, for only 33 gold, Gul can drink enough wine to increase the reward by as much as 325 gold. That almost completely covers the cost of the informant.
Ples is right about the informants themselves – they can be either useless or very helpful. If they're useless, we'll know not to buy them in the future; if they're helpful, that's great. So, I think it's better to buy them anyway.