Awful as it is, this is demonstrably the truth.
How do I know this? Because people have already been doing this, even with 50 gold per hundred bushels as the maximum price for food.
Increase the amount they can potentially make, remove the ability of rural Lords to make gold by any other means, and all you'll end up with is hundreds of !@#$%^&s doing their level best to make as much profit as possible off the cities, regardless of what's good for the cities or the realm as a whole.
This would not be the case if the rural region's survival was equally dependant on trade as the cities are. The reason there's an imbalance is that rural regions don't
need trade for their survival but cities do. If rural regions could starve for lack of gold then rural lords would
have to sell to cities. And if one rural lord is demanding exorbitant prices, another rural lord can easily undercut him. If they get into a stale mate, as you predict, then they both starve, their realm becomes unstable and another realm comes in and takes over. It couldn't be more poetic than a realm destroyed by its own greed. Of course, if a region is starving and the lord isn't doing anything to stabilize it, they should be relieved of their duties and replaced with someone more competent.