The value of a like is not a static number, it is in how you interact with the FB user post-like, and what you do with the information gained from the like action itself (as someone mentioned, "Liking" both exposes your page to the liker's social graph directly in-stream, but also allows you to target advertising directly at those who like you, and/or their friends. VERY powerful stuff.
I would say that the question muddies the answer – A facebook like does not necessarily affect the LTV of a customer directly – but calculating the LTV of the like itself is not only possible, but vitally important for anyone serious about engaging with their target market on Facebook.
What is a Facebook "Like" worth in terms of lifetime value (LTV) of a customer? What is the monetary value of a "Like", both in terms of customer acquisition costs and LTV?