I've just run a quick calculation and can confidently say that a Facebook like is worth $0.007.
I attended a conference at the beginning of the year where the organiser boasted of having 55 000 likes for the event. He took us through a painful powerpoint presentation on exactly how he got those likes, which times of the day were best, whether pictures with text got more likes and shares than pictures without text.
He analysed the social media campaign he and his minions had spent months working on in every possible way and concluded it a success.
When asked how many tickets he'd sold from this Herculean effort he mumbled 'two'. 2 tickets at $200 each from the social media campaign. So 400 / 55000 = $0.007.