For starters, a few of my observations on being a Precog:

-Precognative visions is like Crack Cocaine, I imagine. The more you use it to find out general details about your life, the more you feel the need to do it for everything.
-Remember, as the player you have no idea if you botch. Play that up at times.
*ESPECIALLY if you use the power to master the stock market, or any other highly volitale, long term situation.*
-Take some tips from movies that have examined Precognition. I like Minority Report. Long term operations (like stock manipulations and drug smuggling) are easier to ferret out. They have BIG FOOTPRINTS. Crimes of passions and truly random acts are harder to catch and you get less time respond.
-Take into account that when you do things that change the outcome of a vision that events change around you as you do so The Butterfly Effect.
-Take into account that you are not the only Precog on the block. Sometimes one Precog acting on a vision will cause things in your visions to change unexpectatedly. This is really important in the Aberrant setting were you not only have some powerful Precogs, but you also have your Mega-Smart characters doing things as well.
-Look at the greater effects of your actions. By making yourself a multi-gizzilionaire, you are taking away the money from other people who were supposed to get it. You are interfering in the plans of hundreds, if not thousands of other people. Small businesses die, or never get started. Someone's brilliant idea dies for the lack of capital. The world is altered.

Now, the reason Preston doesn't have a 100 Billion dollars? To begin with Preston was raised in a wealthy Boston family - professional white collar types. His family was more concerned with public service than wealth accumulation.
Then there is a small matter of the Security and Exchange Commission (SEC) and their concern with novas unduly influencing the Stock Markets. Novas moving around large quantities of monies attract attention from all kinds of regulatory agencies. The world is not full of stupid people. If they tracked down Islamic extremists and drug cartels, they can track you down too.
Lastly, there is the very real fact that multi-billionaires attract all kinds of attention all on their own. You are a mark for charities, the enemy of rival corporate concerns, and a worry for world governments. Without a doubt, the Directive, Project Utopia, and INTERPOL have files on you. Better yet, you are very likely to be the target off all kinds of groups with a grudge and guns. Somewhere around the globe your interests are running sweat shops and/or raping the environment. It comes with the territory and the DOTCOMs were a pretty unique fluke ... and besides, all those guys are well known.

Assuming for the moment that Precog and it's like doesn't count as insider trading...

The real question is, why would Preston get 100 Billion dollars?

After a point, money becomes fairly pointless. Troll could earn a Billion dollars a year for a drug company (I'll spare you the math). Instead he gets 2 to 10 million working for PU. Why? Because there isn't much difference between 10 million and 1000 million and because PU offers the research he wants to do.

Preston invested heavily in Russia when it collapsed and Projet Utopia intervenned. All of this was pre-eruption and he made several times over on his investiments. The whole Russian economy rose up lie a pheionix and Preston was a multi-millionair.
Then he erupted and had to seriously rethink what he was doing. He lost some fund disentangling his money's from Utopian porjects, taking some losses. Since then, he had sound and steady investments growning his wealth. He had anywhere of beteen 35 million to 74 million in nontangle wealth, and about 12 million in propertied.
Preston does monitor the City municiple bond groups that manage the citied debt and make sure they stay healthy.