Tampoco hay que pagar nada con las ediciones Express, pero se recomienda que se registren (el registro es gratuito).
Pueden verificarlo en las FAQs oficial:
http://www.microsoft.com/express/support/faq/
How much will these products cost?
Effective April 19th, 2006, all Visual Studio Express Editions are free permanently. This pricing covers all Visual Studio 2005 Express Editions and Visual Studio 2008 Express Editions including Visual Basic, Visual C#, Visual C++, Visual J# (only available in Visual Studio 2005 Express), and Visual Web Developer as well as all localized versions of Visual Studio Express.
How long do I have to register?
You'll have 30 days from when you install your Visual Studio Express Editions.
Can I use Express Editions for commercial use?
Yes, there are no licensing restrictions for applications built using Visual Studio Express Editions.