Really glad that you are trying it out. You don't have to pay anything if you can stick to subdomain. In simple words 'webflow.io' will be attached in your domain name.

If you want to use your custom domain you need to select one of their site plan which starts from 12$/month.

I am currently not using any site plan for my portfolio https://tejesh-ai.webflow.io/ which works perfectly fine as I don't mind having Webflow brand in it.