Both Magento and BigCommerce will require some costs at the outset to build and design your store — which will vary depending on your internal resources and if you use an agency or not (see question above). Beyond the initial set up, to understand your total cost of ownership, you need to first understand what is and is not included in your ongoing costs.
BigCommerce is a SaaS platform, meaning your subscription fee includes hosting, security patching, maintenance, updates and support. With Magento, you will be paying for licensing fees (depending on the version you use; see question above), hosting (again, depending on the version you use), maintenance and keeping the platform secure. It will depend on your specific business needs, but generally BigCommerce stores have a lower total cost of ownership than Magento Commerce stores. If you’re interested in specific pricing for your business, talk to a BigCommerce expert.