That really does vary greatly.... depending on the type of business you are refering to!
Are we talking about different types of Commerce sites....if so, are we talking services, merchandise, products, agencies etc......or are we talking hosting, design, managment, ?????
Be a little more specific, as there are vast worlds of difference between hosting and design companies, not to mention things like E-bay, specialist sercive providers, general produce locators, dating agencies etc.
Here's a couple of things to ponder though - there are obviously a LOT of successful businesses, online or otherwise, out there on topics from cooking classes to web hosting to real estate to dog grooming. I think you can make any of them work - if you have an interest or a passion in the topic, dedicate yourself to the building of that business, work at it consistently, and make sure that your serving customers in some way.
Sounds like you've got a keen interest in making money -- which is great. That's what a chunk of the American dream is all about, right? But, having been involved with several different online projects and businesses, I would make sure that whatever you pick, you have some sort of personal connection to - it'll make it easier to stick with it and make the venture profitable.
Take Dani for example, the owner of this forum. She started with an interest in computers and tech stuff... wanted to bring a community together about that topic... and now look at the forum. Thousands of people... advertisings come flocking... but she had a personal interest in it.
I'll get off that soap box and offer another suggestion -- try finding out what some of the most searched on topics are. That might give you a clue as to what people are out there looking for online -- and I'm sure finding a business to run off it won't be that difficult once you determine that topic. :)