If you for example are a reseller of commodity goods, you can't have a lower price then the price you buy it for. So you can't have a model of say unlimited goods for a monthly fee. And the price will most likely be based on per good.
[1] http://githubengineering.com/introducing-dgit/
[2] I'm a GitHubber. https://github.com/jssjr
Not saying the price is related to the cost, but I am not sure about the no marginal cost part.