Why the interviewer is asking this question: The interviewer is asking to align expectations for the role with your long-term expectations. While interviewers typically want solid alignment on near term (i.e. under 5 years) expectations, more latitude can and will be given for longer term goals.
If they don't want to give benefits to stocker and cashiers they should have part time hours and paid $17.50 per hour no benefits. Also how many teenagers would like a parttime job from 3 pm until 7pm and maybe get the pay they give their cashiers $8 -9 an hour.
Typically, when you run the numbers my ballpark prediction is you'll find you're at least at $20/hour for a single-engine parking lot sweeper and $4 or $5 above that in an hour of running a twin-engine machine. And, remember, that's just for the sweeper.