So, this one might seem a little weird, but bear with me. In settings that rely on a re-creation of the modern world economy, the US dollar or other forms of currency might be used. This is only logical. Still, other kinds of settings might not only require different fashions of currency, they might also not even use money at all (ala Star Trek) or they use things we might not consider 'money' at all.
EXAMPLE: Seeing as how I mentioned I was basing some of my own novel's ideas/plots on a setting very
much like ancient china, I had to be aware of how people paid for ... anything at all! I found out (very interestingly) that silk was so prevalent at a certain point in the history of the Han Dynasty that people actually used it as payment. Sheets of silks were historically a kind of "bond" people paid to other people for goods. I decided an interesting plot point was two different kingdoms use different dyes for their silks (red and blue) to distinguish one kingdoms "money" from another. Each kingdom guards its silk production facilities and the trees that feed the silkworms. This sets up awareness of where/how money is set up. It's not meant to be the biggest point but it is meant to afford the setting some kind of verisimilitude.
In the end, focus on knowing how your characters and how money works. I can't work for free and unless your character happened to inherit a staggering fortune which is held in a goblin bank, know how he/she gets paid. Work from there to see how your character acts when he/she looses his or her job, or when their globe trotting adventure to save the world from space ants has them return to an eviction notice.
