PowerZone's 4-2-1 extractor was RM400 the last time I ask for pricequote. You'll get two for RM800

For me, a step by step would be extractor -> air filter -> muffler -> camshaft -> piping.

The only problem for camshaft upgrades nowadays, none of the manufacturers seemed to bother making cams optimized for automatics.

I put piping for the last is because oversizing the exhaust piping does not do much. You could even feel power loss at lower rpm. Besides, you're on automatic. For automatic, the best performance mod is to gain as much power as possible from the lower till the midrange rpm.

For the muffler, maybe you could try look for 2nd hand Tanabe s-flow.... they're usually priced below RM400. For value setup, maybe you can get an 'F-1' brand muffler. It's kinda 'cap ayam' but my cousin had one of those last time... s-flow type with big 'stomach'. The sound is not noisy. And probably, you can remove the catalytic converter and replace it with bullet silencer to reduce more noise.