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After doing a few sums, I would say you're pretty close.
I guess it's dependant on key popularity. 'casino' keys could result in impressions of 1000+ per day pulling around 40 clicks for $10-15.
Online casinos have got healthy budgets in general and are usually comfortable paying 40cents /click, they tend to get their money back easily from their visitors who are looking to play, but small businesses would maybe suffer from window or comparison shoppers who aren't planning a purchase.
I guess I will give my clients the option, but not push it.