I’d say that the resources gathered by setting a fleet to raiding are roughly comparable to the cash earnt from a business. But it’s a random bunch. Set a fleet to trading,and you’ll get slightly less value (80 to 90%), but you get specifics.
Additionally, as mentioned, the following can affect what you fleet brings in:
Level of fleet.
Any rituals on the fleet (Lure of Distant Shores, Blood and Salt, and others)
Any effects on the area being visited (“traders to Asavea will recieve extra Green iron this season”)
Any effects on the activity (“All raiding fleets are suffering from extra Grendel patrols, and will produce reduced revenues due to doing more fighting and less plundering”)
During downtime, you select what you want to do with a fleet from a drop-down menu. Each option has any special effects displayed, and you can click through them to pick out what you like.
Additionally, there are plot missions that you can send a fleet on, sometimes. Cash reward of these varies enourmously