It seems that after the update that Commercial CSs now build both Commercial Hub and Harbor, Trade routes become very profitable.
City Center 3 gold, CH 3 gold, Harbor 3 gold=9 base gold
A single trade route through water/railroad doubles this to 18 gold
With other policies like Triangle Trade, or Suzerain Hunza or Antioch or Kumasi, or bonus from great merchants or Great Zimbabwe, the base gold can easily yield 30-40.
And under the current patch, such gold can be further multiplied by Colonial Tax, Kilwa Ksiwani and Casa De Contraction or Amenity bonus, making this 65% stronger.
So a trade route can yield as much as 60-70 gold per turn, crazy. Even with only Triangle Trade and Colonial Tax you'll yield 22*1.25=27.5 gold per turn.