For me, I paid $1495 for biix(dot)net. Most of the time I don't buy the .net version of my active business, but for some of the more complex ones, it makes for a great test environment.
Once you have a serious business going you need to have a separate development environment, so I'll typically use a .net or .org.
The reason most businesses will buy the .net is to protect their brand from competitors. Either within their industry or confusion with a brand outside their industry.