Red Door Yoga is a good option if you are practicing Ashtanga. Rob, the owner of the studio, follows the traditional way of adjusting your postures which may feel like pair-gymnastics but in fact helps you to deepen the practice. Private classes are 600rmb a session, but alternatively, you can also sign up for one of the morning self-practice sessions (Mysore) and cross your fingers that no one else turns up. Drop-in classes for those group sessions will cost you 150rmb each time. Note that since this one follows the traditional Ashtanga practice, the place is closed on full moon and new moon days.