I was curious how to rule the availability of buying passage on ships or caravans for players. I can figure out how much it should cost to do so, but not how frequently ships and caravans run between settlements with trade routes between them.
Any one have any ideas?
I think it's like any other good or service. There is a chart for availability by market class at the head of the equipment section. E.g., if the trip from your market class V village to the distant big city costs 20 gp per person you should find someone willing to take you to your specific destination in 1d4 months (25% chance per month),
if it's closer and it only costs 10gp there's a guy who does the trip once a month. And there's always someone going to the nearby (1gp or lower fare) market town that's on the way.
Frijoles Junior's method is very similar, if not identical, to what I've done in my own games. The only thing you'll need to decide is what the smallest increment of cargo you'll allow somebody to request, and whether trying to move a large party of PCs, henchmen, mounts, and mercenaries constitutes a single large booking of passage or several smaller ones.