I won't hire those tutors who is currently studying in UNI if he/she only charges less than HKD100. Even a part-time cleaner would charge HKD60+ per hour.
I will not use university student in HK, esp. for English. Their English standard is pathetic. So I would say anything under $200, you really need to watch out for their standard unless you just want a person half babysit and half teach.