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I would suggest that you go for google adwords rather than facebook.. if it's linked in then i guess it's ok. there are a lot of experiment conducted how effective facebook campains are and if you base it on the interaction of the audience who like your page then it would be just the same as the audience you get if you purchase "like farm" outside of facebook.
Anticipated ROI
Anticipated ROI = (Anticipated Revenue from SEO efforts – Proposed Cost of the SEO Project)/Proposed cost of the SEO project
Actual ROI
Actual ROI is the one which eventually decides the fate of your future engagement with a client.
Actual ROI= (Total E-Commerce Revenue through SEO + Total Goal Value through SEO) –
cost of running the SEO campaign/ cost of running the SEO campaign
Read more: http://www.optimizesmart.com/roi-calculations-seo/#ixzz3h4F0PtU3