Yes, WooCommerce is free for WordPress in its core form. The official WooCommerce site states that it is free to download and free to modify, and WooCommerce also says it costs nothing to add Woo to a WordPress site.
However, running a store may still involve other costs. You may need to pay for hosting, a domain name, premium themes, paid extensions, or third-party services such as payment gateways, shipping tools, or marketing add-ons. WooCommerce itself is free, but extra features can increase the total cost. So, the base plugin is free, while advanced store needs may require paid tools.