ERPNext is Free and Open Source ("free" as in "free-speech"), but the services around ERPNext need engineers to support you. These are what is charged. Since scale, complexity and support requirement of a deployment is proportional to the number of users, the Enterprise Support contract is also priced per user. Also most ERP products are priced per user, so it is easy to compare with other products. ERPNext Support is a fraction of what you would pay for NetSuite, SAP or Oracle Cloud.