Una cita directa de la página de descargas de software de Oracle.
Software Downloads
Free to download, free to learn, unlimited evaluation time
Developers: All software downloads are free, and most come with a Developer License that allows you to use full versions of the products at no charge while developing and prototyping your applications, or for strictly self-educational purposes. You can buy products with full-use licenses at any time from the online Store or from your sales representative. http://www.oracle.com/technetwork/in...ads/index.html
En conclusión, cualquier cosa que se bajen con fines educativos, es gratis.
Pd, La edición Express no existe más en la 11g, una pena, ya que era muy fácil de instalar.
Saludos