What exactly is the bonus for capturing an enemy capital, I thought I read somewhere about 8,000, is that a one time bonus or do you get bonus daily>
There is no financial bonus for capturing an enemy's capital. One, it would be too easy to exploit, and two, it would make a player who's already winning more and more powerful very quickly.
There is a penalty for losing a capital, though. Any player who doesn't control his capital has his income cut in half each night. So, combined with the fact that a capital can always be used to build a base, they're definitely still worth capturing.