Is it truly native English speaking teacher? Where does he/she come from? What's his/her teaching experience? With HK local kids or experience claimed to have in overseas? That's very different.
Frankly speaking, there're many so called Western teachers, there're different classes. Teachers from local international school definitely charge a lot higher, as they have experience and proven record. For others, it will depend on your luck. However, looking at the surface, I wouldn't expect a solid quality for HKD300 every 2 hours.