Hello,
As per the amendment in Sec 55(2) of income tax act the cost of acquisition of certain assets is to be taken as NIL if there original cost cannot be determined or cost of acquisition of self generated assets are to be taken as NIL.The following are the some of mentioned assets:-
(i) Goodwill of a business.
(ii) Trademark or brand name associated with the business;
(iii) right to manufacture, produce or process any article or thing
(iv) right to carry on any business;
(v) tenancy rights;
(vi) route permits;
(vii) loom hours.
Your case falls in the category of clause iii as patents is nothing but the right to manufacture, produce or process any article or thing.
So Cost of acquisition in your case is to be taken as NIL .