You could use 'costliest'.
Adjective: Costly
Comparative: Costlier
Superlative: Costliest
When you are looking for similar words, you can find synonyms/antonyms on websites like thesaurus.com or merriam-webster.com etc, and use its superlative (or any required form). For more insight on how to make superlatives, check out degree of comparison of adjectives.
