Here is an infographic I found on the website of Percolate about the real cost of social media marketing.
It gives a pretty clear view about how much each components of a social media campaign costs. What is not put into prospectis is the reach you may have with this strategy. These figures are based on the Nestlé marketing campaigns, and I believe that depending on the size of your company, and foremost the size of your online community, the cost/reach will be very different from one account to another. Anyways, I would also like to see the comparaison between these figures and traditionnal media such as radio, TV etc... to see actually what is the most cost efficient.