Mono is open source and free software. Because of the "single static binary" policy (no dynamic linking) of app stores you have to obtain a proprietary license from Xamarin.
• The C# compiler is dual-licensed under the MIT/X11 license and the GNU General Public License (GPL).
• The tools are released under the terms of the GNU General Public License (GPL).
• The runtime libraries are under the GNU Library GPL 2.0 (LGPL 2.0).
• The class libraries are released under the terms of the MIT X11 license.
and:
When do I need to obtain a license from Xamarin to the Mono Runtime?
We only require licensing for uses of Mono and Moonlight on embedded systems, or systems where you are unable to fulfill the obligations of the GNU LGPL.
• The C# compiler is dual-licensed under the MIT/X11 license and the GNU General Public License (GPL).
• The tools are released under the terms of the GNU General Public License (GPL).
• The runtime libraries are under the GNU Library GPL 2.0 (LGPL 2.0).
• The class libraries are released under the terms of the MIT X11 license.
and:
When do I need to obtain a license from Xamarin to the Mono Runtime?
We only require licensing for uses of Mono and Moonlight on embedded systems, or systems where you are unable to fulfill the obligations of the GNU LGPL.
source: http://www.mono-project.com/docs/faq/licensing/