It'd be nice if it were possible to set up preferential trading prices, according to the trader or lord's home region/duchy/realm. I might want, for example, to make sure the prices are good enough to pay for our trader's long missions to far-away lands (to keep up with mercenary escort cost, caravan cost, scout cost in addition to the actual food costs), while not really wanting to pay as much to foreigners from right next door who don't have any of these costs to cover. A bit of protectionism would sound reasonable to me.
I completely agree; this would be an extremely helpful feature to have.
Maybe food prices could be fixed by treaty?
This is much trickier to code than it sounds.
Also, it will add further layers of complexity into the already complex food-management interfaces, which will cause even more people to just write them off as too confusing to bother with.