Well, I was fortunate enough to be playing before the Caravan nerfs, so I have around 1.5mil growing steadily in the bank. In my opinion, though, if you wanted to get a sizable sum of money fast and easy, just loot villages. Make sure you save before in case you get caught, of course. At least, that's what I did in the original Warband, dunno if it's still as profitable. Once you have a decent amount, I would personally recommend either pursuing a storyline, or starting a rebellion with one of the wannabe kings. It's surprisingly easy to get nobles on your side (remember, save is your friend!), and once you have a couple of em you can begin taking over castles and cities. In the case of a rebellion, you are able to appoint any castles and cities taken to whomever you want. Give yourself a city and a few villages, maybe a castle as well, and you'll be raking in gold from taxes, and you'll gain a fair amount of renown to boot.