Registering for Shopify is free. You can sign up for an account here: https://www.shopify.com/?ref=mrktr
If you're thinking about starting a store on Shopify, there are a few things you'll need to take into account: the monthly subscription fee, the cost of your domain name, and any other Shopify app fees you might want to add on.
The monthly subscription fee for Shopify starts at $29USD. If you decide to use Shopify Payments, there are no additional transaction fees. If you use a third-party payment gateway, there is a 2% transaction fee.
The cost of your domain name will depend on where you purchase it from. Shopify has a partnership with GoDaddy, so you can buy your domain name directly through Shopify. Domain name prices start at $11.99/year.
There are also a number of Shopify apps available, which can add functionality to your store. These apps typically have a one-time fee or a monthly subscription fee.