There's an indirect way to know how much money you get from raiding total : on the Finances page, your total income is broken down between taxes, trade and "other".
Other = king's purse + tribute from protectorates/diplomatic deals + raiding trade lanes
(king's purse is the flat amount of money you'll make every turn no matter what. It varies from faction to faction, the single-province factions for instance have a high purse so they can buy large armies. Major factions generally have 3000)
So if you want to know exactly how much money you're making from a given raid, check the finance screen, remember the existing number for "other", then raid and reload the finance screen (it doesn't update if you keep it up). The difference is what you're making from the raid.
As for how that number is calculated, I have no idea. I always assumed it was a fixed percentage of the road's value, the percentage being a function of the number of ships rather than the kind of ships that are raiding it, which would mean itty bitty galleys and Indiamen are the best raiders... but they're also easy to chase away, so there's that. That being said, again, I have no idea how it
really works, so I could be talking out of my arse there
