Orginally posted by Lawgiver

Orginally posted by Chaos Lord Arioch
Also there should be a random element added to the land routes as there is with the sea routes (eg. foul weather, missing caravan?)
It may sound like it complicates things but I think its better than saying, "OK, you have a land route, every quarter you receive an extra 5 GB".
A caravan would be treated like a ship or a unit of soldiers, with a maintenance cost and everything. It's very simple.
For the 3e rules underdevelopment we have added maintenance costs to trade routes and discussed several methods for adding randomness to GBs generated. However, we wish to keep things simple and reduce the number of rolls.

As far as caravan's comin gup missing I would leave that to the DM's discretion and random events as warlord_nabron suggested.
Why are you complicating things by adding a maintenance cost to a trade route? The Gold Bars produced would account for the maintenance cost or are you trying to make it possible that a trade route loses money. That would just take a minor adjustment in the GB production charts and then every holding should have a chance to lose money for its owner.