These three costs behave very differently, so it helps to see them separately. The initial conversion is a one time effort that depends on the size of the site and the number and quality of drawings, compute depends on how the twin is used and how often it is updated, and users scale with how many people need access.
Separating them makes it easier to understand what you are paying for and to plan for growth, such as adding sites or users over time. It also makes it clear what happens to the cost after the first year, once the conversion is done and the ongoing costs are what remain.