How much profit does an online store make?

That a lot like asking "How much does a red car cost?" Are you talking about a Ford Focus or a Ferrari?

It depends on a lot of things: what industry you're in, what sort of competition you have, the demand for the product you sell, how likely your customers are to shop online, how you've differentiated yourself from your competition (ie do you have a strong brand, do a lot of people know you, do you currently have a brick and mortar location, etc)... There are a lot of variables.

Your best bet is to talk to a web developer and see what they can tell you about your business. Feel free to check out Zodiac Creative (http://www.INVENTtheUNIVERSE.com). We'll be glad to offer you the advice you need.