Copper Coins:
- Halfpenny (lowest possible denomination)
- Penny, equals 2 Halfpennies
- Half Groat, equals 2 pennies
- Groat, equals 4 pennies
- Star, equals 2 Groats or 8 pennies
Silver Coins:
- Stag, equals 7 stars or 56 pennies
- Moon, equals 49 stars or 392 pennies
Gold Coins:
- Dragons, equals 30 Moons, 210 Stags, or 11,760 pennies
The conversion rate is 1 Groat = 1 US dollar
We assume that Essos and Westeros use the same currency
This might seem insane because when Ned Stark became the right hand, he learnt that they would host a tournament which offered 40,000 Gold Dragons to the winner of the joust, 20,000 to the runner-up, and 20,000 to the best archer. If we did the math, that would equal a total of around a quarter billion US dollars of tournament rewards. That's just what the Game of Thrones Wiki states, so I will work around that.
While this is what I will follow in regards to currency rate. It won't be an important aspect of the story. I'm writing a story about a knight, not a merchant!