Hello! I have an easy way to determine a rough estimate for the site. First you need to understand whether it will be a commercial product or a regular private page. Commercial products require complex certification and compliance with safety regulations. This is a lot of work and requires a large team of professionals. Therefore, look only at those companies that offer turnkey commercial projects. Be sure to compare all the sites in your portfolio. I can give you an example of an Australian web development company - https://www.shtudio.com.au/services/web-development/ An online store requires not only a beautiful and convenient design, but also good hosting. Consider all this in your future expenses. The more nuances are taken into account at the planning stage, the less accidents there will be at the store release stage.