From Dag Homboe's blog, a spreadsheet to help you in calculating your ROI in regards to your Social Media plan.
Dag says" For this research, we identified 13 parameters that play a role in determining the ROI of Social Media. It is not necessary to include all parameters in a ROI calculation – pick the parameters that make sense."
Dag also goes to the trouble of explaining thoroughly his reasoning in the build-out of the spreadsheet, and also how to best use it.
Social Media ROI spreadsheet