Please review the Mendix plans and the FAQ sections as they provide good guidance on what your users are and the limits. The cost will increase as you go over the user limits for your specific packages. But this is where you will need to plan for your load and what you expect to be doing.
As the docs stat, if you have very specific questions relating to the cost, contact mendix support. If you just want to know your user limits and how they are defined check the FAQ in the page link provided. Scroll down to the bottom.