I'm hashing out the following idea before I submit to the suggestions... This is the initial idea, so there's a lot of room for improvement I'd imagine.
Copper, Iron, Silver, Gold coin forging specific to kingdoms...
Once players have a claim, allow them to forge coins specific to their kingdom which then goes on to effect vendor prices.
When selling, you have the following options...
Vendor Options:
"Other Kingdoms Coins"
When selling items, you can allow vendor to select "other coins" with increased item prices... possibly have an option to reject other kingdom coins entirely.
Example: I have 10 gold pieces from Kingdom A, but I'm in a town in Kingdom B that charges extra for "foreign coins" which means items cost more for these coins.
OR
"Kingdom Coins"
When selling items, you can reduce prices for coins of your own kingdom.
Example: I only have 7 gold pieces for Kingdom B, but the vendors in Kingdom B are charging less if you use their own currency.
OTHER NOTES:
Add a "Banker" building that allows you to switch foreign coins or raw materials for "kingdom coins".
Each kingdom can have a specific code and when the coins are forged they add this specific code to the coin. It seems like it would be easy to program into the game since you're adding essentially a single ID variable to the kingdoms which is extremely easy for the server to serialize. Then when the vendors are used just have an "if" check to see if the kingdom id of the coin matches the kingdom id of the vendor before presenting a player with a list of prices.
SYSTEM IMPACT:
This would allow for a very dynamic and interesting currency system that may give more purpose to using currency instead of just bartering. If there is a very successful trade city then they can create an incentive to use their currency instead of other currency which would make for a competitive economy with other kingdoms.
Example:
Person A: Hey, I have this job for you and it will pay 10 gold coins from Kingdom A.
Person B: Hmm... I'm actually looking for Kingdom B coins. Can you pay me in those?
Person A: No sorry, I don't have any of those.
Person B: Well, I guess you should get some sucka.