You can estimate costs for a social media campaign by analyzing the costs and time of the people responsible for managing the campaign, as well as what percentage of their time (and expenses) is used to maintain the social media activity. If you spend time generating content for websites/blogs or other social media sites, then this time needs to be captured as well, as it is never free. You may also want to factor in the lost opportunity of the person or people who may be pulled away from another activity in order to engage in the social media activity.