You would need a lot of margin to not base the price on the production costs.
If 200 users cost double as much as 100 users, it's hard to not base the price on number of users.
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.
Not saying the price is related to the cost, but I am not sure about the no marginal cost part.
[1] http://githubengineering.com/introducing-dgit/
[2] I'm a GitHubber. https://github.com/jssjr