The cost of building an online store with Magento can vary widely depending on your specific needs and the scale of your store. Magento Open Source is free to use, but you'll need to consider costs such as hosting, domain registration, SSL certification, and potentially a payment gateway. If you need to hire a Magento developer or an agency, this can be a significant part of the cost. Custom designs, extensions, and integrations can also add to the cost. For Magento Commerce, the platform itself comes with a monthly fee which starts at around $2,000 (as of this writing). For a custom-built Magento store, costs can range from a few thousand dollars for a basic setup to tens or even hundreds of thousands of dollars for a large, complex site. It's also important to budget for ongoing costs like maintenance, updates, and marketing.