Costly is a synonym of Expensive. Expensive is related to Money. you are likely to find Money in/at/on Pocket. Pocket is derived from Poke. 
Example is related to Protest. Protest is related to Authority. Authority is related to Judge. Judge is related to Law. 
Costly <--Synonym--> Expensive <--RelatedTo--> Money --AtLocation--> Pocket --DerivedFrom--> Poke 
Example <--RelatedTo--> Protest <--RelatedTo--> Authority <--RelatedTo--> Judge <--RelatedTo--> Law 