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][1] or [merriam-webster.com][2] etc, and use its superlative (or any required form).
For more insight on how to make superlatives, check out [degree of comparison of adjectives][3].


  [1]: http://www.thesaurus.com/browse/more+expensive
  [2]: https://www.merriam-webster.com/thesaurus/expensive
  [3]: http://www.worldclasslearning.com/english/adjective-degrees-of-comparison.html

However, please mind the context while using antonyms/synonyms. [Please see @Luke Sawczak's comment below.]