Golarion (i.e. Pathfinder) has a couple of plutocracies you could check out for inspiration:

Katapesh - The Bazaar of the Bizarre, Katapesh has one rule above all others: "Do as you will, but do not interfere with trade." You can buy anything there - from weapons and mercenaries, to rare items and components, to drugs and slaves, and the ruling council doesn't care as long as the dough keeps rolling in.

Druma - The Merchant's Paradise, this one has stronger religious overtones to it. The chief religion/philosophy there is a set of teachings called the Prophecies of Kalistrade - in a nutshell, they believe that if you follow their teachings and devote yourself to business dealings (rejecting most deities in the process), you'll amass vast wealth, and flaunting that wealth will cause you to become even more favored, thus gaining more wealth. It's a bit of a blend of meritocracy and plutocracy, since clearly anyone who has succeeded deserves to be wealthy and therefore deserves to be in charge of those who haven't, and anyone who can outwit a higher-ranked follower in a business dealing has the potential to surpass them. Followers of the PoK amass wealth but have more restrictions, such as foregoing more hedonistic pleasures, even refusing to touch anyone who doesn't also follow the Prophecies.

Some common themes between these two that you can leverage:

- Amassing wealth is the highest form of virtue, so long as you do so in a way that doesn't inhibit trade.
- Morality is either barely a concern or not a concern at all to what is considered legal, hence all the slaves and drugs.
- The letter of the law trumps its spirit - loopholes are both a common and expected part of business dealings.
- Crime that violates the letter is not tolerated; if you're going to steal from someone, you have to find a way to do it legally.