After digging around in the game data and doing some testing, I'm pretty comfortable with the way that trade routes between cities are calculated. I hope this will be helpful to everyone trying to decide when building a road between cities becomes profitable. The same calculation is used for all difficulty levels, but playing at anything below Prince reduces your route costs. See the modifiers section for more detail about this. Revenue Calculation: Trade route income is calculated based on the size of the city which is being connected to your capital's trade network. The size of your capital city is not currently used in the calculation and can be ignored when deciding to build a road or not. Trade route yield is roughly equal to (Connected Non-capital City Size * 1.25). This appears to be true even when your capital city is smaller than your connected city. For example, from one of my test games: Kyoto(4) to Osaka(4):Trade Route Income 5.01 Kyoto(4) to Tokyo(5): Trade Route Income 6.26 Kyoto(4) to Satsuma(2):Trade Route Income 2.51 Roads: Roads cost 1 gold per turn in upkeep...so if you need to build roads on ten tiles to connect your city to your trade network, you wouldn't break even from that expenditure until your connected city reaches size eight (8 * 1.25 = 10). Keep in mind that a longer road may still be profitable to build if it allows your trade network to be extended to multiple cities in another part of the map. Harbors: For remote cities, harbors may be a better investment than roads provided that at least one of your cities already within your capital's trade network also has a harbor to complete the link between cities. Harbors have an upkeep of three gold per turn. goodolarchie posted a cost analysis that compares the investment of buying a harbor outright with the number of turns needed to break even here. Trade Route Modifiers: I haven't tested all of the modifiers yet, but I'm listing the following based on the game data and/or documentation. Let me know if I'm missing something and I'll add it to the list. Great Wonder - Machu Picchu: +20% more Gold from all Trade Routes. Must be built in a City within two tiles of a Mountain. Requires Currency. Leader - Harun al-Rashid: Trade Caravan trait, +1 Gold from each Trade Route Policy - Commerce/Trade Unions: Maintenance paid on Roads and Railroads reduced by 20% Difficulty - Settler/Chieftrain/Warlord difficulty levels reduce road and railroad costs by 66/50/25 percent.