As already mentioned in one of the Journal entries, I think that caravans are greatly misbalanced at the moment. Currently it is most effective to simply build all caravan routes. This way your food production is more or less doubled and your guildar income increased a lot.

Therefore the number of caravans that you typically want to run is  N * [(N - 1) + M] where N is the number of cities in your empire and M is the number of cities of Neutral/Allied empires.

Considering only a medium sized empire of 5 cities and allied with a 3 city empire that is 35 Caravans and it is growing quadratically with the number of cities. For a larger empire this leads to a map really cluttered with caravans. For instance i had a game with 9 Cities and an neutral trading partner with 4 Cities resulting in 108 Caravans running.

On the one hand this leads to a dramatical increase in guildar (in my example the guildar income was more than doubled) and additional food income which also increases quadratically with the number of cities. This easily offsets the administration cost. It further results in the fact that income produced by the cities themself is only a minor contribution to your gold income. On the other hands this also leads to massive lag, especially on older machines.

I would propose to reduce the number of allowed caravans to only a few per city, and rebalance the economy on this reduced guildar income. The limit could either be fixed, and depend on some Diplomacy research, or it coul be capped at the level of the city. This would further encourage the building of larger cities instead of a number of small ones.

And yes, I specifically build cities without any resources to make a point.