Hola,
Could you provide a few (more) examples of the use of the verb 'to cost' something?
What sorts of things does it cover (or could you use it for everything)?
Everything fluctuates in price, so are we talking about things that we concentrate our daily lives on (sometimes obsess about!): stock market/ currency, houses, petrol, food, drink? Things we think of as fluctuating day to day?
Gracias,