Try to avoid the word ‘cheap’ when discussing online communities. Online communities are not cheap. They can get rather expensive.
You have to plan a community, engage with the audience, tailor yourself to what the community wants and hire people to run the community. It’s time consuming, slow and expensive.
But cheap web traffic isn't the point. You’re not trying to find the cheapest web traffic. You’re trying to find the most valuable web traffic.
You’re not trying to find someone that is willing to click a link to read some information. You’re trying to find the the most engaged, enthusiastic and loyal group of people possible.
Developing an online community is the most cost-effective means of online marketing, not because it’s inexpensive, but because the outcome is so valuable.