Hi @vishav, the freely available Tobii Core SDK may be used to develop applications that use eye tracking for Gaze Interaction Use.
However, if you are intending on using the Tobii 4C Eye Tracking for eye tracking research (ie long term storage of gaze data) then you are required to purchase a special licence in conjunction with the Tobii Pro SDK to do so.
You can find further details of the Tobii Pro SDK @ http://developer.tobiipro.com/
In the meantime, for pricing options, please send a request to [email protected]