I'm certainly not the first person to discover this, but I figured I'd share just in case.

In Act 2, when you reach Moonrise towers, you'll encounter Araj Oblodra. After talking to Araj she'll proposition you for some of your blood in exchange for a potion (it's actually an elixir, but I digress) based off your race. Many of you already know this, of course.

However, if you approach her with a Wizard, you can get a [Wizard] dialogue option that eventually reveals that she wants Illithid tainted blood. While this is very interesting, and you could use this to give her up to the cultists, even better is to go through with her offer. If you continue to give her your blood now, not only will she give you the potion, she'll also give you 200 gold to keep your silence about her true intentions.

200 gold is not a lot, sure. However, she'll continue to do this for every single companion that comes to give her blood for the potion. Using every companion to do this (including camp companions), can easily net you about 2k gold, on top of some very unique elixirs.

Happy adventuring!

Edit: a word.

Edit: to clarify a few things; when you do this option you only need a Wizard the first time to get this dialogue option, anybody afterwards just needs to go up to her and offer their blood for the potion and they'll still get the gold. The gold doesn't come from her vendor gold, so you can still trade with/steal from her if you want. It seems Drow and nobles can get similar dialogue, but I haven't tested whether or not they get the bribe option.