It's possible to launch an ecommerce site for less than $1,000, depending on the features you need for your business to succeed. Since there are many different aspects of an online store, you can pick and choose exactly what you need and only pay for those items.
Additinally, It is depend on whether you want a readymade script or a system/platform/marketplace from scratch. Building a system/platform/marketplace from scratch require lots of time and money so buying a readymade script is better option for small scale startup businesses.
If you want to start a multivendor ecommerce marketplace then Yo!Kart "http://www.yo-kart.com" is one of considerable option for you. At $999 you will get a fully customizable ecommerce platform with advanced features. Also you will get In-built APIs with multiple payment gateways.
Feel free to discuss or to share your queries.