My deceased mother had missed some Salaat due to her extreme condition of illness.

Answer

Payment of fidyah for undischarged Salaat is Waajb (compulsory) if the deceased had made a wasiyyat (issued a directive) to pay the Fidyah (Compensation). If your mother – May Allah Ta'ala grant her Maghfirah and Jannatul Firdaus – did not make a wasiyyat, then it will not be incumbent on her heirs to pay Fidyah for her missed Salaat. Nevertheless, good and concerned children should try to make amends even if their parents did not make a wasiyyat. If the children pay Fidyah on behalf of their deceased parents and make dua of Maghfirah for them, it is hoped that Allah Ta'ala in His Infinite Mercy will accept it and forgive the deceased.

The Fidyah for a missed Salaat is the same as the Sadqah Fitr amount. Whatever the Sadqah Fitr amount is at your end, use it as the yardstick to calculate the Fidyah for your mother's unfulfilled Salaat. If you do not know the exact number of Salaat, make an estimate. Fidyah is to be paid for the Witr Salaat as well. If the amount is beyond your means of affordability, then give whatever you are able to. Fidyah money must be distributed to only Fuqara and Masaakeen (poor Muslims).