I guess they really look at your ability to fly the plane rather than your academic performance.
That is not true in SIA context. Before a cadet starts his basic flying training, he has to go through 6 mths of intensive study and sit for exams in order to obtain ATPL. Some cadets couldn't make it through this stage. And I dare say from my own experience that if I had put in as much effort in University, I probably would have gotten 1st class honours. (Most cadets especially those in Seletar now will probably agree with me on the toughness
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Any advice for me on what I can do to show that I have a strong interest?something that doesn't cost too much as I'm not exactly loaded. Asking me to enrol in some flying school is out.
Getting your own PPL is just one of the many ways to show interest. Most of us cadets had 0 experience when we started. Check out my previous post for my advice. I can't tell you specifically what to do because your interview experience and mine will not be the same.
But just to give you an example, if someone is interested in cars, you can expect him to be able to differentiate between a Ferrari and a Porsche w/o looking at the logo, right? So can you tell a B777 from an A330?