Fidyah (expiation) of one salah is 1.575 kg wheat one can pay either wheat or moderate market price of it. If you have missed so many prayers then calculate 6 salah of every day and get the total quantity of wheat as per the total number of salah and pay the fidyah accordingly. It should be given to the poor and needy who are entitled to receive Zakah. You can give it in a madrasa accommodating poor students. It cannot be spent in mosque, building madrasa or in giving salary.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband