The "Precious Metals" quest gives the largest cash reward in the game I think, 58 gold (plus a Crow Dagger) if you have persuasion up to the 4th tier. Alternately, the quest will yield only 48 gold (and no dagger) if you are less persuasive. The only problem with Precious Metals, however, is you need 40 gold seed money to start it properly. You also have to answer the dialogs correctly or you could end up with a lot less. If you screw it up with stupid answers, you could actually lose your 32-40 gold seed money, making it potentially one of the worst financial disasters of the game.
To ensure you don't screw it up you must have completed the following prerequisites:
(1) You must not have killed Godwin during the Mage Circle rescue mission when you discover him hiding in a armoir while fighting your way up to the area where you will be thrown into the Fade.
(2) Your persuade (coersion skill) should be at tier 4 to get the best reward.
(3) In dialog, always be friendly and use the most advantageous persuade when it pops up.
(4) If your persuade is not at tier 4, do NOT use persuade to reach for the best reward. Not only will you not get it, but you may cause the quest giver to disappear and never come back. I know for sure a tier 3 coersion skill level will get you the second-best persuade rewards, but can not say for persuade levels less than 3.
When starting the quest with Rognak, if your persuade is at least 3 (but possibly lower, not sure on that tho'), you can persuade him to lower his security deposit from 50 to 40. Since the default buy offer by Godwin is 50 later, you basically won't earn anything if you don't use persuade at all. So you really must succeed at your persuade at the start or you probably won't get anything later.
The buyer, Godwin, must be questioned in the right order or you concievably could lose all the money you invested (although you could sell the smuggled lyrium to a merchant for 12 gold or so, but it's far less than you put out). The dialog permits you to screw up. So do not threaten Godwin with blackmail at the start, - doing so will net you 8 gold but he will run away and you will lose your 40 that you invested. To max out your return you must first play it straight, deliver the goods but persuade him to pay a higher fee. After getting your ordinary payment, then you blackmail him.
Start with Godwin by selecting the dialog that you "know a dwarf..." in common. Do NOT start the "why do you need lyrium" or "you now deal with me" dialog yet as that leds to the blackmail, which will cause him to disappear leaving you with (12 + 8) - 40 = negative 20 gold for your efforts. Continue with the transaction normally. The high persuade (level 4 coersion only) reward follows the dialog where you suggest he pay "75" gold. Godwin won't give you that much, but he will give you 65 plus a Crow Dagger...if you succeed at the persuade. If your coersion level is 3 or less, then suggest he pay "60".
To get to the blackmail dialog after you conduct your original business, talk to him about the political affairs of Ozammar after you do the original deal. Then ask him why he needs lyrium. Godwin will then talk about the need for illegal smuggling. Then make suggestive comment, "you must make a lot of money off this". When he questions your motive, say, "Because I'm curious, and I saved you." Then you must use persuade (coersion) to succeed at the "Share some of that wealth or people will find out." With that Godwin will give you an extra 8 gold.
You must then return to Rognak to pick up your messenger fee. He of course correctly remembers it as 10 gold. Do not attempt persuades unless you have the minimum levels or have previously saved your game in case you screw up. With a coersion of 3 or less, you can get 20 gold more. With a coersion of 4, you can demand 30, but he will give you 25.
If you have been following the gold so far, this is what you can get doing it right:
(-40) + 65 + 8 + 25 = 58 gold (plus the extra dagger Godwin gives you). Starting with 40 gold, you end up with 98, best quest reward in the game. A 245% profit.
If you totally screw everything up with stupid dialog, here is what you pay:
(-50) + 0 + 0 + 0 + 12 (from selling the smuggled lyrium to a merchant) = -38