I don't know any "fair market values" for it but I'd definitely have various tiers. Starting with a larger expense for a "starter" tier since that would be the hardest/most time consuming (building a social presence from scratch). Then it could taper off into monthly subscriptions maybe with follower milestone bonuses type things.
Since you deal with mostly smaller businesses it's a bit harder to come up with a good price for you but also not out pricing yourself for a small business. You might be better off including it all in a package along with the SEO stuff. But charge at least enough where you can make a profit after hiring someone yourself to do it for you for each campaign/company. That way you can take on a lot more and let those people specialize in that company/niche.
I don't know of any because I haven't looked into it at all myself but maybe see if there are any well known companies that already do this for the larger corporations and try to find out their rates lol. Could also be useful to show your clients the success, as far as customer interaction and such, those various campaigns have for those companies and why they need it too.