Lords charge what dukes are willing to pay. On Atamara, there's a large food surplus in my area. Due to the surplus, it's rare to find a duke who's willing to pay the maximum fee. As a result, Lords have to adjust their rates accordingly and their food is usually sold at around 30/100.
Dwilight is another matter. My realm on that continent has a very slim surplus, and there's steep competition for food from neighbouring realms. Consequently, it's easy to find offers for 50/100, and only generous Lords are willing to sell for less.
It's the law of supply and demand which drives food prices. 50/100 isn't extortionate at all, in fact I consider it a very moderate price. I could easily see desperate Lords willing to pay 75/100, or even, in extreme cases, 100/100. Those would be extortionate prices, but 50/100 is basically just a middle ground, it's suitable for realms that are in need of food, but not necessarily in dire straights. I actually think that the maximum price should be raised, as that would make trading more dynamic as well as provide more incentive for Lords to participate in it. I'm unsure of why the cap is so low to begin with.
