According to this: http://support.microsoft.com/gp/lifean45#sl5
Silverlight 5 will be supported for 10 years. Not many software vendors are prepared to do that.

Actually, that is innacurate. If you want to develop for .NET, all you need is a text editor and the .NET framework redistributable (which you can download for free). If you want to use a rich IDE, you would need to purchase Visual Studio .NET.

As an alternative, I've heard that there are a couple of open source IDEs for developing on .NET which are also free(ie. SharpDevelop for C#).