Alright. $100/month is very small for a marketing/seo budget so you have to be very careful on what you spend it on.
Assuming you've done a keyword analaysis of your target market, your focus should initially be on content. Spend the first week or two doing nothing but getting content onto your site. Make sure you've got something different or unique that sets your site apart from the rest.
Offering something unique is one of the key factors here because a low budget means you're going to want alot of return visitors.
When I did this, I spent the first $300 on link development. I outsourced the linking campaigns to a reliable company where they guranteed 50 inbound recriprocal links each month of PR1-3.
During these three months I did nothing but build content and other features on the site.
After 2 months had past, the site developed a PR of 2. It had a total of 90 recriprocal links. The links weren't super-high quality and their link pages had 80+ outbound links on all of them. But the PR2 of the site was enough.
With the PR2, I cancelled my link deal and spend all my time requesting link exchanges with sites myself. My site had alot of unique content (because I had typed it all from scratch for 2 months) and alot of PR1 sites were more than happy to exchange links.
At the end of the third month my site jumped to PR3 (I managed to get about another 50 inbound links, most of which were PR0 or 1).
The next three months I spent on buying text links on high ranking sites (PR6+). $100/month didn't get much, but it was enough to get a large boost.
At 5 months my site was exchanging links with a large number of sites and I was starting to recieve link requests from sites. My PR was 3 and slowly climbing.
At 7 months I was generating 300 unique visitors/day from the search engines and text links that I bought. It was enough to sell some ad space on the site and generate some additional cash, which I sank back into advertising.
At 9 months I was getting 500 uniques/day and the number was climbing. It now had a 3 advertising spaces on the site and was dumping all the money generated back into advertising/text links.
At 12 months the site was getting 1200 uniques/day and it appeared to level off at that. I was showing up on the 3rd page of Yahoo for most of the keywords, and the 5th page on Google. Most of the traffic came from the advertising I had setup.
But at those 12 months I had managed to start taking money out of the site. I took 25% each month to start paying off the debt I incurred to start the site up, while the remaining amount went back into the site.
After 12 months it was standing at a page rank of 5, but not very good search rankings (that's what I get for trying to get into a popular market lol). I'm sure I could have pushed harder to get better search rankings, but I think I did pretty well.
This was one of my first sites. And while I've done better with other sites, I still think about this site a fair bit since it was my first real success. But by success, I mean I was able to draw about $300/month from it out of the $950ish that it was generating.