Generally, it is seen in any job offer letter or offer of appointment under the separation clause (Notice of Termination) that "you shall be serving ___ months' notice to the company, should you seek separation from the company or ___ months' salary in lieu of the notice period."
This is a general clause; whether it is one month's notice or one month's salary OR three months' notice or salary depends on the company and the position. Thus, it is written as salary, and salary need not be basic salary alone; it often includes DA, HRA, etc.
Therefore, it is not wrong to say that the shortfall in the notice period is calculated based on the full monthly salary and not just on the basic salary.