Most wiki software does not cost anything, because it is Free and Open Source (FOSS). There are a few wiki engines which are commercial distributions, requiring you to pay. Also there are an increasing number of hosted wiki solutions where you pay to set it up on their server.
These commercial offerings are normally available for free initially, with an evaluation licence which runs out after a time, or after you expand the system beyond a certain number of users. Sometimes prices are scaled according to number of users registered on your system.