For revenue < $300K, you are looking at a one-time license fee of $2700 for the first platform and $4500 for subsequent platforms.
libRocket uses the time-tested open standards XHTML1.0 and CSS2.0 (while borrowing features from HTML5 and CSS3), and extends them with features suited towards real-time applications.
Can be integrated in any engine (or application) since it relies on you to provide the backend, including rendering, texture loading, etc.
The AntTweakBar library is free to use and redistribute. It is released under the zlib/libpng license.