Folks be MUCH more aggressive with Merchants:

Use them to actively attack the opposition merchants - it doesnt even matter if they are on a trade resource or not - if you take them over you get a LUMP SUM from "acquiring" their business e.g I've just taken out a French level 3 merchant who was wandering near to my borders, he wasnt on a resource - but acquiring his business netted me a lump sum of 1100 florins (this is on h/vh fwiw).

So nab the other guys businesses then trot back to you ownl ittle resource, or go on extended walkabouts taking over any merchant you find and can "assimilate".

The ENTIRE equation for Merchants, as to whether they are worth their 550 florins cost, is completely re-written when you use them aggressively and only hog a resource when they've got no available targets to take over.

And bear in mind that that 1100 florins wasn't much, I *think* i netted over 6k from acquiring an Egyptian merchant in Antioch (didnt remember to keep ane ye on the previous balance, but it was certainly plenty of dosh)..