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.