What are you expiating?
For fasts that cannot be made up (chronic illness, old age): feed one needy person per missed day.
Cost of feeding
What you owe
Method & sources
Fidya for missed fasts: Qurʾān 2:184 (feed a needy person per day). Broken-fast kaffāra: ḥadīth of the man who broke his Ramadan fast — free a slave → fast 60 consecutive days → feed 60 (al-Bukhārī 1936, Muslim 1111). Oath kaffāra: feed/clothe 10 or free a slave, else fast 3 days (Qurʾān 5:89).
Fidya for those who can fast only with hardship — feed a needy person for each missed day («wa ʿalā alladhīna yuṭīqūnahu fidyatun ṭaʿāmu miskīn»).
The man who broke his Ramadan fast — the kaffāra ladder: free a slave → fast two consecutive months → feed sixty needy persons.
The same incident in Muslim, confirming the broken-fast kaffāra.
Expiation for a broken oath: feed or clothe ten needy persons, or free a slave; whoever cannot, fasts three days.