It will depends. I'm a software developer and have worked on many e-ecommerce web applications and even developed my own for my own e-commerce web site.
I have billed out anywhere from $200 - $8K per month.
I'm always improving my e-commerce web site. Adding new features like "text me when my order ships", adding new icons for pinterest and instagram.
Updating the homepage with a new main graphic and adding new products.
In 2003 - 2005 I worked for a company as a full time employee. I was their web application developer and I was making around 40K per year and they had over 10M in sales. I created their e-commerce application from the ground up and it is still being used today!