Nochmal zum Handelsbonus der errechnet sich so:
"The amount of trade and the bonus payment you get for establishing a trade route between two cities with a caravan or a freight depends on quite a few factors. The formulas below contain all the details.
Trade = ( trade of home city + trade of destination city + 4 ) / 8
Note that it does not directly depend on city size or distance. The following (cumulative) modifiers apply:
Both cities are yours -50%
Freight instead of caravan +50%
Cities connected by road +50%
Cities connected by rail +50%
Cities on different continents +100%
Airports in both cities +50%
Superhighways in home city +50% "
Quelle:
http://www.civforum.de/showpost.php?p=4870&postcount=11