Another possible solution to this problem, a problem which I've
noticed before now, is to keep the usual set up rules and whatnot,
keeping everything in the hands of the guilder. However, the crown
always has to give permission for a route to be set up, and can shut
it down at any time (by having guards stop and seize all goods
relating to that particular trade, for example). The crown is then in
a position to negotiate for a hefty chunk of the profits from the
route, as there is little the guilder can do to stop it. On the other
hand, the ruler can't afford to get too carried away, as less profit
is better than no profit if the guilder decides that the route would
not be economical.

My 2 spice caravans' worth.

John.

"Once I was a lamb, playing in a green field. Then
the wolves came. Now I am an eagle and I fly in a
different universe."
"And now you kill the lambs," whispered Dardalion.
"No, priest. No one pays for lambs."
- David Gemmel, Waylander