How much will E-Commerce software cost?
Depends on which one you pick.
Most good ones are actually free (open source).
Try OpenCart http://www.opencart.com/
There are lot of e-commerce softwares available in the market. The price of the software depends upon it's features. The SAAS based solution starts from $30/month. Most of the companies are also offering their ecommerce software to the end-users as a complete package. The cost of that package ranges from $2000-$4000. So, choose a plan which is more convenient for you.
Those replies seem to have answered a very vague question as well as possible, so thread closed.