I think that most of the time in Astrum, food is passed around for free. By default Libidizedd is always buying for 20/100, so anyone that sends me food by caravan gets paid for it. (Higher prices posted for foreign sellers, as needed.) But mostly, we just move it all around whenever it needs to go.
Personally, that's one of the things I really like about Astrum. We are mostly united as a realm-based team, rather than a bunch of people standing asking "What's in it for me?" Gold, food, etc. is all (usually) directed where it needs to go, and not based on how much profit can be gotten out of it. Part of that is, of course, the unifying effect of the common faith, and the shared goals it provides. But even beyond that, most of us just aren't into the whole "me me me me me!" thing.
In D'Hara, I promoted the purchase of food at good prices since quite a while, maybe even since before I was duke of Paisly, back when I was Marquis of Paisland, but I'm not too sure.
Rergardless, it was never about conflict in our case. When I became duke, I voluntarily hiked my buy prices all by myself, both to be more fair to the rural lords whom I needed, and especially to attract foreign traders and exporters. Since one can't offer different prices for different people, these two went hand in hand.
It probably makes me the poorest duke in the world, but it has kept my city fed in almost all of the winters and built good relations with my vassals. After all, a starving city would be no richer anyways. Wouldn't want to end up like Port Nebel!