For shirts or blouses:
(1) Brooks Bros. - wait until their sale that you may be able to get one at around HK$350. The slim fit one is better.
(2) Zara - range from HK$150 (basic style plain color) - HK$450, quality quite good except their sleeve are too long that you need alteration. Alteration charge is around HK$35. They have quite a lot of style and they are good for tucking in your pants.
(3) Stage of Playlord or Playlord - usually quite stylish and not all of them are good, depends on the design of each season, I bought at around HK$300 during sale period.
(4) Giordano Ladies - not recommended as the quality is bad and sometimes style looks strange.
Pants
(1) Zara - Though I am 5' 2" only, but you may find one at least with their various sizes and styles. However most of their pants are low waist. If you don't mind, they are quite nice. I need to make alternation each time when I bought pants from Zara. Price ranged from HK$129 to HK$600.
(2) Playlord - Their pants are more slim and if you are looking for this style, this is a good place. I usu. buy it when they have at least 50% discount, i.e. net at around HK$300 - HK$400.
Other than the chain store above, I find there are some small shops in Causeway Bay Kimberly Center, almost the highest floor. They have a few white shirts, dark color skirts and trousers for sale. However quality is not very good and price is not too cheap also. A shirt cost you around HK$300, skirt and trousers are almost the same price.