The structure of a kaffara is more revealing than any single instance of it, so it is worth reading the oath case closely, since it is the one the Quran sets out at fullest length.
A person who binds themselves by an oath and then breaks it owes an expiation, and the expiation is stated as a set of options rather than a single requirement. Feed ten poor persons at the average of what you feed your own family, or clothe them, or free a slave. Whoever does not find the means for any of these fasts for three days. The closing note is that this is the expiation for oaths that have been sworn, and that oaths should be kept.
Three features of that structure carry through the other cases.
The first is that the standard of the feeding is set by the offender's own household. It is not a fixed sum, so the same fault costs more from a wealthy person than from a poor one, and the law does not have to be amended as prices change.
The second is that the fast is the fallback rather than the norm. Where means exist the expiation is directed outward, at people who need food or clothing, and only where means fail does it become a private act of self-denial. The other Quranic cases follow this pattern, and in the case of zihar the sequence runs from manumission to a fast of two consecutive months to the feeding of sixty poor persons.
The third is that a kaffara is owed and is not a devotion. It can be neglected, and neglecting it leaves an outstanding obligation. This is why the jurists treat kaffara alongside debts and vows rather than alongside prayer, and why the technical questions it raises are questions of law: whether the feeding may be discharged in money, what counts as consecutive in a two-month fast, and what a person does when an option named in the text is no longer available.