The same could be said for a lot of quests - the cash rewards aren't all that high compared to what you earn by regular gameplay.
The requirement to woo a governor's relative is, in fact, the entire reason for this quest's existence! The whole idea of either marrying or charming a governor's niece or daughter is borrowed from a much older game, Sid Meier's "Pirates". In that, once you'd either charmed or married a niece or daughter, she would regularly tell you the current locations of the treasure fleet and silver train. Rather than try to match this, I decided to write a one-off sidequest to get the silver train.
I might increase the amount of looted silver, though. Never mind that you need to be fairly advanced to be introduced to a governor's relative; you're going to face a couple of squads of soldiers, all armed with muskets, so you'd better be good at melee combat! So I probably don't need to worry about giving excessively high amounts of loot to a low level player character.
Having said that - where did you sell the silver? Find somewhere where silver is an import (check the "Colonies" tab of the F2 interface) and you'll get a better price than if you sell it where it is an export. Having the maximum "Commerce" skill plus all the commerce perks, either on your own character or your quartermaster, should also help.