i was complaining to hubby that i was getting NO random events in my current game. at all. my first random event literally happened to bismarck, a wedding between our nations he picked the give money for a present i guess, since he now has a +1 towards me, but it wasn't an event for me, it was for him. i had nothing, more nothing, a mine destroyed, then lots more nothing.
then i finally got something. something really really spiffy!
so that castle was worth it

i built it just for the trade route and spy 25%, didn't even know this was possible. yeehaw! if i hadn't had the 267g i could have gotten a bit of free money, or 2 free pikemen ... i'm glad i was relatively rich, that would have been a bummer to miss out on.
Sephlock if there's a cap it's based on size and trade routes, but there's definitely not a rule that every city will be the same. you don't actually have to travel to the cities to see how much you'll get. if you've seen the cities before, you can check while you're safe at home. with the merchant active, hold down the shift key and right click on the destination city. hover over the trade route button -- it'll be greyed out, since of course you can't do it remotely, but it will show you the amount of gold you'd get. cancel that order before he ever starts to move, repeat for another city, etc, til you find the best bargain.