What about updating the chart to show price modifiers instead of some absolute costs?
Every herb would be assumed to have the same base cost, but then modify that cost by region herb grows in, region herb being purchased in, scarcity, effect, special prep prior to purchase, etc.
Then if your base currency is copper, apply the modifiers and get your costs. Same for gold.
This way, the costs have a common, repeatable construction and can easily be ported between currencies.
The resulting chart may not be pretty though.
