Heather-
Here’s a perspective from someone who doesn’t do offline promotions very much:
1- You could take the $100 and invest it in getting a good book on SEO. SEO Book by Aaron Wall is good, as is The Unfair Advantage, although I think SEO Book is a better value for the money. Even if you don’t buy a book on SEO, have a look at seobook.com. It’s Aaron’s blog, and it’s usually packed with good info. If you can get good search engine rankings, you don’t need an ad budget anymore.
2- You could create a huge list (1000 or more) of keyword phrases that have 3 or 4 words that are targeted to your site. Overture still allows online gambling ads on their foreign sites, so you could market to the UK for example. With a list of 1000 keyword phrases, if you bid Overture’s minimum bid of 10 cents per click, then if you get 1 click per month per phrase, $100 will buy you 1000 clicks.
3- Most online gambling message boards have a billboard or spam section where you’re allowed to post your links for free.
Hope that gives you some more ideas.