It depends what the e-commerce software is for and the ability of the admin user.
Open source software has a lot of strong contenders, but it depends upon the complexity of the site and how much support is expected.
If you want instant support if things go wrong, expect to pay for it. If you have the ability to go onto the Internet and seek out a solution for yourself, then maybe open source could be good (and cheaper) for you.
Personally, I like Open Cart and have had considerable success with it. But there are many others, both free and paid for.