I think the OP meant that this is the bottom line margin you need. As an Auction Marketplace, with highly motivated prospects, searching for what you offer, on

I think the OP meant that this is the bottom line margin you need. As an Auction Marketplace, with highly motivated prospects, searching for what you offer, only a click and a buck away, and an extremely low barrier of entry, I would argue that Adwords is as close to an efficient marketplace as possible, and represents the high end of the cost per customer within the framework of profitability.