I'm looking for numbers regarding Facebook ads - If I spend $100 on Facebook ads, how many users will see my ad? how does the system work?
Facebook gives you an estimation of number of Likes, you will get by promoting your page through facebook ads. I won't recommend facebook ads, I would rather advise, you should talk to the owners of pages with massive number of likes. They will charge you money but you will definitely get more likes than the facebook ads. I myself promote facebook pages and I have huge pages for this service. In $100 you can find a decent amount of Likes for your FB page, so don't waste your time on facebook ads, as it is costly and less effective.
I have to disagree with Valeed. Facebook Ads are a goldmine if you use them in the right way. You will not have an accurate answer to your question. The number of fans that you will get depends heavily on the quality of your ad and your conversion rate. If you are newbie to Facebook I recommend that you get fans through free methods such as forum signatures, organic search engines, or other social media. You can even comment on posts and interact with fans of other pages that are similar to yours. If you still want to use Facebook Ads then I recommend to take Amy Porterfield's course. Amy is one of the leaders when it comes to Facebook and she will teach you how to use Facebook Ads.