The cheapest pay as you go gyms are the public ones. It is IIRC about $22 a visit or $180 a month no contract. I go to Sheung Wan and it is a well equipped gymn with good space. The catch is you must do the course first - 3 hours and a test. There is some other way to do it but look that up on the Fitness and Leisure site.
It is simple and the stretching exercises are participative so you learn it by doing. It was in Cantonese but they could answer questions in English and there was no need to ask as he showed the right way to do stretches and how to use each machine correctly. I thought I didn't need it but I did learn and the instructor was technically perfect. The questions at the end are like T or F on something like no pain, no gain. T or F? They want F as if you are hurting yourself you are doing it wrong.
This way no contract and you can pay as you go.