This all depends on the size of your store. If you are just starting out you can get a feature-rich store on Shopify for $29 per month. You'd be surprised how many features they provide. For small to medium sized business, Bigcommerce is also a good option. If you are a bigger company, both Shopify and Bigcommerce have enterprise-level options as well. There are also some free options like WooCommerce, which is a plug in for Wordpress.