m0n0wall is under the BSD license, which basically means that you can do whatever you want with it (including modifying and selling it) for free, as long as the original copyright notice and license appear somewhere in the documentation and/or the software itself. There are no warranties of any kind though.

For the full copyright notice/license text, see http://m0n0.ch/wall/license.php.

Although you don't have to pay anything for m0n0wall even if you sell it, if you do find yourself making money by selling m0n0wall-based products, a donation would be very much appreciated.