It depends on the business. A big franchise, like owning a McDonald's can be over a million dollars. Something smaller, like a Subway is around $15,000. You can buy a website that has small traffic for a few hundred dollars, but if it has a lot, it could be millions.
I think the best and most fun businesses to own are the ones you start yourself from scratch, which costs almost nothing but your time to get going. Many huge companies, like Apple Computer and Microsoft got started this way.