Hi sinhr,
There is no cost for using QST or the QST SDK. The QST SDK was released under the license found here: http://software.intel.com/en-us/articles/intel-quiet-system-technology-intel-qst-sdk-software-license/. There are two different licenses described there; One for an ISV, OEM or IHV, and the other for the End User. The ISV license allows the ISV to modify copy and distribute software as long as it is released under a license that has terms at least as restrictive as the above linked agreement. I'm not a lawyer and I can't definitively say all that the license means, so I would suggest that you consult with your legal representative. The End User license does not allow any copying,modification or reverse engineering.

Hope this helps,
Judy