According to Redhead, you can take plates, such as silver plates given as quest rewards, to the Imperial City where it can be traded for ten thousand gold coins.
To put it into terms that can be understood. I put a gold coin at around ten yen, so a silver plate is worth one-hundred thousand yen whereas this quest to escort the princess had a reward of ten million yen. I don't even have the slightest concern about taking the money of the Royalty, even if it is taxpayer's money. The nobility just spends it on indulgences anyway.