"El libro de ejercicios cuesta mucho dinero."
Traducción:The workbook costs a lot of money.
ok, much se usa cuando el sustantivo es incontable, pero solo se usa con las oraciones negativas o en preguntas.
That textbook does not cost much (money). Does this coffee cost much?
si no sabes que sea correcta usar much vs a lot of vs many, usa a lot of. a lot of básicamente funciona siempre.
I reported exercise books being wrong and duolingo have emailed me that it's now an accepted answer. However, looking into it, workbook in American English is more "textbook" in British English. Although this sentence seems to translate to "exercise book", está muy fatal. It is very wrong in many ways because exercise books are for doing exercises (you write down on an exercise book) but you don't write down on a workbook/textbook.
American/British: Workbook/Textbook (libro de texto, libro de ejercicios) Notebook/Exercise book (cuaderno)
It's called textbook because it contains text. It's called exercise book because that's the book you use to attempt your exercises.
I assume los estadounidenses lo llaman workbook porque es el libro que tiene el work, o sea la tarea que necesitas hacer. y notebook es el libro donde you can take notes.