@Viafique The initial building of an eCommerce site is free, but to launch the site and accept payments, you would need to purchase a premium plan. The eCommerce premium plans range from $27 to $159 depending on how basic or advanced you need the plan to be. Since the prices for premium plans may vary depending on geographic location, the best way to see your actual pricing is by pretending to upgrade a site (you can go through the steps and then just don’t complete payment).
Here is more information that may help:
Choosing a Premium Plan
Wix eCommerce