You can setup an ecommerce website for cheap today by installing an ecommerce store script that you can get for free and install on a web server.
I would start out with a VPS server to host the ecommerce website with and then move onto a dedicated server once your ecommerce website takes off.
You can get a VPS hosting account for around $20.00 per month but be aware though that you need to know your way around servers to operate a VPS server since it's more involved than a shared hosting account.
Then after getting the ecommerce store script and server you need a domain which is around $10.00 per year and an SSL certificate to secure the server for people to make payments on your website.
So to begin with an ecommerce store you can get started for around $50.00 per month in the beginning.
When you begin to get more website traffic and sales you can expect to pay around $200.00 to $300.00 per month but you should be making enough in sales to cover the hosting cost and domain fees by the time you spend $200.00 to $300.00 per month.