Subscribing to Odoo
You can use one Odoo application for free without subscribing. If you wish to install more applications (as you likely will), you must subscribe to Odoo in order to keep using their Enterprise cloud-hosted version of the software.
For each user, the current pricing is $25 per-month at the time of writing. Each application you use will also incur a monthly cost, which varies by application.
Below is the Odoo Online Pricing calculator for February 2018 in the United States with one user and three applications.
You can locate the Odoo Online Pricing page at https://www.odoo.com/pricing-online.
Depending on your requirements, an Odoo subscription might be a good decision. Installing and maintaining an Odoo installation takes a degree of expertise and has risks for production systems. You must maintain adequate disaster recovery procedures in case of server crashes or hard drive failures. There are also complexities in applying bug fixes and migrating to newer versions of Odoo. This book will help you with many of these tasks. It can be quite convenient to have an Odoo subscription so that you can focus on the functional, rather than the technical, aspects of working with Odoo.