You could use 'costliest'.
... and the prettiest package is the costliest one.
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.
However, please mind the context while using antonyms/synonymssuch word-for-word lookup. [Please see @Luke Sawczak's comment below.]
