Expensive. Dear. Pricey.
Costly.
Dear,costly and high-priced are some of the synonyms for expensive.
Synonyms: dear, expensive, pricy. Antonyms: cheap, inexpensive.
High-priced, pricey, expensive.
The superlative for costly is "most costly" or "costliest."
They are synonyms.
Synonyms
Yes, costly is an adjective. E.g. It was a costly mistake. (One that was expensive to fix, or that led to other bad consequences.)
Relative and kin are synonyms.
It should be most costly.
They are synonyms.
synonyms