Get a domain set up with content and invest on promoting it or buy a good website at a low price and if theres anything left over, use it to promote the site with adsense or affiliate marketing.
I would buy articles or forum development services and kick off a new blog/forum with the help of the writers. or forex
$100 buys a lot more online than in real world, you can buy several hours of service, a small traffic website to flip etc.