Grabbing a Twitter handle, setting up a blog or creating a presence on a social networking site can be completely free.
Of course there will always be a cost in terms of time and resources and maybe you will invest in some tools to make your social media presence more effective. Many businesses think that social media marketing is cheap.
Again, it can be but if you want to do it well there are more investments to make than merely time and tools. You will have to invest in people, processes and resources to monitor the conversations, respond to them, create valuable content, participate, analyze the impact of your marketing efforts and integrate the social media in your overall marketing processes and strategies.