Hi w00ti, I'm from Singapore too, and let me also add a disclaimer that I'm also an instructor, but my advice is based on my own experience, as well as lessons learnt from others. As DrBill wrote, there are MANY other factors besides price that you MUST consider. Do realise that all businesses have overheads to maintain, and yes, plenty of operators out there will undercut each other just to land your business. But something has to give when it's that cheap, and it's usually instructor to student ratio. It's not uncommon to see one instructor to more than 10 students. Educational institutions are very guilty of this because they want it cheap. So what gives besides the high ratio? Using "assistants" who are usually NOT trained to look out for signs of panic, or how to react to situations. Imagine yourself being at the end of a line of, say, 10 students, and your instructor is at the other end. Heaven forbid, if there's no one looking out for you, and you're feeling panicky, who can you turn to? Even in a pool situation, when an instructor took their eyes off the student, the student bolted to the surface and after a series of mishaps, drowned.
There is an excellent article that is pinned on Scubaboard, called "how to look for an excellent scuba class". Do search for it.
As I've written numerous times on my own blog: if a dive operator tells you that "it's only diving, how hard can it be?", run as quickly as you can. Yes, call me an alarmist, but I tell my students that diving IS dangerous if you're not well trained, and GOOD training takes time and money. And diving safe isn't just about "getting over and done with" the training, it's about mindset as well. I've been in some hairy situations and while I griped about it during my course, I was thankful when the **** hit the fan, and I was able to react to it quickly and instinctively, because even during leisure dives, I took a bit of time to practice emergency procedures.
Diving is supposed to be fun, but it's fun only when you're well trained and have the peace of mind that you'll be able to take care of yourself if something happens. Don't scrimp on your training.
Enjoy your journey!