All PHP files (with WP code) must be distributed with a GPL license. Images, stylesheets, and JavaScripts do not need to be and can be sold commercially.
To be clear, you may still charge for the PHP code it however must be distributed under the GPL license and no additional restrictions may be made. If maxfoundry is distributing the entire package as GPL (including images, js, css) then their current license pricing may not hold up as the single/multi site restriction should violate the GPL.