The pricing structure of Lithium, Jive, Salesforce, Telligent, and most community platforms today is typically based upon monthly or annual usage tiers.
In theory, this means you begin at a low level (usually an estimate based upon existing traffic to your website) and gradually pay more as the community grows. In practice, you’re more likely to be transitioning from one platform to another and stay within a single pricing tier.
The objective of tiers is to make the community affordable to more organizations but create a ‘lock in’ to the platform as your community (and presumably your investment in the community) grows.
For most external communities, the usage tier means the level of traffic (often unique visitors). For internal (employee) community platforms this is the number of registered users (usually a few dollars per user per monthly). Though some platforms (HigherLogic/Salesforce) use different structures.