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.
The average hourly wage for cashiers at Sam's Club is slightly higher at $9.36 per hour. Target cashiers average $8.18. Of course, that doesn't mean Costco employees are being paid like they work at a company like Facebook or Microsoft. The average hourly wage is around $21. That's not a lot considering that Facebook interns can make double what the average hourly worker at Costco makes in a month, assuming a 40 hour work week.