25 Gold (not 25%!?) seems... very lackluster. I’ve certainly had games where in the industrial and later I’m earning in the upper hundreds or even thousands of GPT. For securing every copy of a resource across the globe, I’d hope for something more than half an extra trade route.
Well, you get 25 gold per turn for the monopoly and then another 5 gold per turn for each civilization that you're trading that resource to. Or something like that. That's not bad, really. You're already getting lots of other benefits from the monopoly.