No personal experience but I read in SF somebody tried a tailor in Worldwide house which offers competitive price.
Otherwise, how about tailors in Lippo centre? If you look for budget shirts maker, rent must be cheaper.
I tried two small shirts makers in Causeway Bay, I mean the shop with sewing machine inside their place, but the result was mixed at best.
Not cheaper than Jantzen and not better or sometimes worse than Jantzen.
I wish you a good luck with your...