I don't think so. The cost of running something like Mailchimp isn't in having a couple of servers and some bandwidth, I imagine; the cost is in dealing with spammers, overzealous spam-fighters and people who'd rather hit "report as spam" than unsubscribe. Number of subscribers is not a bad metric.

ETA: Also see bjonathan's comment at http://news.ycombinator.com/item?id=2170619: Amazon, which probably offers something closer to "server and bandwidth", is much cheaper.