Fidya is paid instead of fasting, for someone who cannot fast at all now or in future (e.g. chronic illness or old age) — one day's feeding for each missed fast.
Default of 10 is a rough placeholder for feeding one person for a day — replace it with your local Fidya rate (many local scholars/fiqh councils publish an updated figure each Ramadan).
This is a simplified estimate for a common Fidya calculation (missed days × one day's food cost per person). Fidya applies specifically to those permanently unable to fast (chronic illness, frailty due to old age), not to fasts that can still be made up later (Qada) — see the Missed Fast Calculator for those. Please confirm the applicable rate and ruling for your situation with a qualified local scholar.