Low cost doesn't cut it. You need to invest a large amount to obtain a decent return, in your situation i would probably recommend google or partnerships with large forums/websites.

A partnership would get you a text link or small advert on their site, this will drive more traffic to your website and also help to increase brand awareness. Google on the other hand (adwords) is a very powerful marketing tool and could bring you a vast proportion of your business.

Have you thought about some seo on your website? It's free and can be worth it down the line!