Question

What is an appropriate response to ¿Cuánto Cuesta?

Answer 1

First ¿Cuánto Cuesta? means How much in Spanish to answer correctly to that answer you say a number for an example:
Girl : ¿Cuánto Cuesta?

Guy: Tres

you may saw the object you are talking about.

But if you wan to answer as saying in terms like how much you love someone you say todo esto which means this much.

Answer 2

First subject then Cuesta (certain price)

Ex: ¿cuánto cuestan tres libros? Tres libros cuestan 12 dolares