Some of the best methods of advertising SFI from my personal experience are ,
1. Search Engines
2. Directories
4. Craigslist (For those staying in USA)
5. Safelists
6. Traffic Exchanges
7. Classified Ads
To be honest, advertising cost in any of these channels is either zero or very little.
Results amazing because of the company's repute. People love to jump at the opportunity.
Each one of these sources command a very huge membership and visitors. As a result, as per numbers game, advertiser should have large visitors. Even a small percentage of these visitors getting into this business means, you have added to your organisation which means money for you.