It depends whether you are using an existing CMS to set it up. Today most hosting companies offer cheap ecommerce solutions (which however aren't always usable for you or your clients).
Hosting is also a cheap commodity nowadays (starting from as little as $4/month), which will allow you to host a Zen-Cart, OS Commerce, CubeCart, Joomla or WordPress based ecommerce website cost effectively. However, if you need control of server settings, the price of hosting will go up, because you will need dedicated server hosting. Godaddy.com, hostgator.com and oneandone.com are all good hosts for these purposes.
Another option is to get an esources membership, you get unlimited hosted ecommerce websites, and access to the web's largest database of verified trade wholesalers and drop shippers
Hope this helps
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