Once you have gotten a sizeable fleet, you can start hunting notorious pirates for bounties. Each bounty isn't worth all too much, but it seems to be greatly multiplied when you collect multiple bounties all at once.
4-5 bounties can be upwards of 150,000 gold, where as each bounty individually might be worth 10,000. 7 bounties can be up to 1,000,000, 9 bounties up to almost 2,000,000.

Patarimas:

The highest for silk that I have seen is 4,800. Just so it is known. I also know that for the one bounty I earned 614,000.

Kodas:

The bar tender does say that the port aouthority sells 25 crates of silk per bounty, but the number of you turn in at one time also increases your total bounty. I turned in 52 bountys at one time at a port where sild was selling for 3900. 25 crates of silk at 3900 a peace would be 97,500.
52 bountys at 97,500 each would be 5,070,000. However, I was paid almost 297 times that amount for a total bounty of 1,505,587,200. I have a screen shot to prove this. So the first poster is right. If you turn in multiple bountys at one time, you get paid ALOT more.