If this was an hourly position, that should be advertised and openly available to candidates. Basically a priori information.

If, on the other hand, this is salaried (unlikely) I could understand the firm feeling a bit yucked by the candidate asking this so early. That's something a candidate in most cases can research in advance.

But, again, I imagine this was an hourly position and in such case should have been advertised to candidates.