(Sorry if this has been proposed before)
Traders/nobles with caravans have to pay to gold to keep them, so Lords that want to send or try to get food with them should have to also. This could also make it easier for Traders to have better chances to make a profit.
Lets say example where a Lord can easily sell his food to a city for 40 gold per 100 bushels. There is no need for a Trader. Though what if to do that it cost 10 gold, so the Lord still makes a profit for 30. A Trader could come by and buy it for 35 gold then sell to the city for 40. Thus the Lord makes more gold, and the Trader can make money better.