Good question. I had to cold email a lot of blogs and websites for a long-tail ad network, and I must admit, after some tweaking and spinning, I was able to get a response in some 10% of the emails sent (of course, the first filter you turn on is WHO to cold email). And the concept behind has to be: let's start a dialogue. But to give some perspective, my intention was to pay them for their inventory, and if you intention is to get their money, then I believe the response may be way, way lower.