I've been wondering about this some - I've found that when I have a land-locked capital running bureaucracy and with Oxford, and several coastal cities, building a bunch of harbors and custom houses... the cap gets stuck with domestic routes. Now, some of the bases on those are pretty good, cause I can get size 20+ cities with good corps, but they don't have bonuses for long-term peace or foreign trade or overseas trade, so the yield on them isn't as high. A recent game, I had 4 3 commerce trade routes at the end, while one of my coastal cities had 6-8 commerce per route. Some of that was harbor and customs house, of course, but even without those, they were getting twice as much commerce as the cap. I was losing 20-30 bpt from this, overall. Granted, when we are making 2000 bpt, 20 isn't all that big a deal.