They are asking roughly how much money you expect for the job, to see whether your expectations are roughly in line with what they expect to pay.
If your answer is "well it depends how long my working week is going to be" then sobeit. You could ask at that stage but as a former Personnel Manager I wouldn't be very impressed. Better to get to interview to determine such things.
It's a bit like saying "I don't know how much money I'd expect until you tell me about the pension scheme - and by the way how many days holiday would I get". Its too early in the recruitment process to be asking sich things.