ERPNext is a fully open source ERP software. You pay absolutely zero for running and managing it by yourself.
However, if you do not have hosting capability and are looking to host your ERPNext site with us, Frappe Cloud is the way to go.
Frappe Cloud provides hosting services to all ERPNext sites. For minimal plans based on data storage and server consumption, Frappe Cloud charges and supports you with hosting services . ERPNext subscription comes into play, if you need value added services such as bug fixing on priority, consultation services, customization support, Devops support for migration & upgrades.