Outside of classwork, how much experience do you have with cataloging? If this is your first experience, I would be inclined to not ask for much (certainly not more than ten dollars an hour), given that they would be doing you a favor by letting you work there and learn as you go. When I did a cataloging internship (6 hours a week for about 3.5 months, in addition to my regular full time job that was primarily circulation), I flat out asked to volunteer as an unpaid intern. The experience was extremely positive and helped land me my current position as the sole cataloger at a public library. So, if you want experience, you shouldn't do anything to jeopardize that.
That's just my two cents...others may certainly disagree!