wheel - 12:40 pm on May 25, 2011 (gmt 0)
I'd probably go with a couple of decent directories (though not sure you can get decent directories at that price).
Or spend the $100 on taking my wife out to dinner and then go find free ways to get links.
I work with some niche bloggers. The most effective way I've seen to promote a new blog is to get in with the established bloggers (comment knowledgeably on their blog, offer guest posts, etc). Then hopefully they'll start referencing your blog. I recently watched a blogger promote his site successfully this way - despite being in a competitive and suspicious niche, and despite the quality of content on his blog being questionable.