Is theAI mean? Earth quakes in Timbuktu

I do think, that even the least valuable resource, should be able to pay off the price of a merchant in 10 turns. This would include average levels of skill development in the equation from a starting level of 0 finance. But basically, it shouldn't take more than 10 turns on any resource at all for a merchant to pay off their investment. Otherwise, it doesn't seem like the merchant themselves would want to get involved in trading whatever it is... he is a businessman after all and DOES want to make some profit. If it can't make that level of money, it shouldn't be on the map because no one would bother sending merchants for it. I think this would benefit the AI because their merchants would be making them more money and thus make them more challenging.

Cash cows? I'm not so sure... but honestly, past a certain point, I think merchants should make the difference between simply being able to garrison your cities and defend your borders, and actually being able to launch offensives.

There is another piece of the economic puzzle there though. Armies in other nations should cost significantly more in support costs than ones within your own borders... and armies maintaining sieges should cost even more than that. Armies in recently conquered territories should also cost the same as ones on foreign soil, for 5-10 turns or so. Sort of the time it takes for them to turn from being an occupying force to just local troops. There would also be a good mechanic to make "steamrolling" harder, by making it take longer and be more expensive to integrate a new territory.