I really struggle with this one. I have a professional job and like to dress slightly above business casual. I do a capsule wardrobe and try to buy quality fabrics and classic styles that will last for years. I do have the occasional look in thrift or consignment stores for pieces, as this would have a big impact on my budget, but I find that when pieces are cheap, I end up buying things that are not-quite-right just because they're cheap, and that feels like a waste of money when I don't wear them, even if it's just a waste of 5-10$.
In Canada (like Aus), clothes are not cheap, but I try to buy exclusively on big discounts (40-50% off), and I agree that Costco is at times great value. I also cycle my clothes from work clothes, that as they wear become leisure clothes, and then become garden clothes, and then become pajamas, lol.
All that said, I still tend to spend around $300-400 per season (fall-winter, spring-summer) just to replace things that are wearing out. I shop for the best fabrics I can at the Gap (on sale), especially for pants and summer clothes, a department store here that carries Lord & Taylor merino sweaters and drapey tops (on sale) for work, and Costco for tops, pants, socks when there's something good. A few years ago, Costco had a merino sweater in a perfect cut for me for about $35--I bought 3 and am still wearing them regularly.
That $600-800/year budget doesn't include shoes, though, which are also expensive, even for mid-range quality. I find I can only get about 3 years out of a sole/cushion support, even if I spend $100-200 before my feet start to really complain. I had some extra foot issues this year that meant replacing EVERY pair I wear. Sigh. So this year, I've spent about $600 on shoes alone--even with sales! Hopefully that will do me for a while!!
And that's only me, not my husband, who wear jeans and t-shirts year round, but wears through things faster than I do. I budget $100/month, which feels completely extravagant and ridiculous compared to a lot of the numbers around here.
I struggle because lots of my colleagues are dressed down compared to me; there are no expectations that I dress as I do, but after much trial and error, it's what I feel comfortable in. I love my clothes, love my capsule wardrobe. But there's no question if I was working from home, I would save some cash! I am contemplating spending even MORE for some specific pieces that I might get more years out of. I can't seem to get more than 2 years out of my tops, so I'm always replacing a few each season. Ditto for pants--I might get 3 years out of some of those, but usually need to replace at least 1 bottom piece per season.
Honestly, I really don't feel like I'm buying all that much; I tend to do a blitz 2-3 times a year to replace what I need and I never shop in my daily life. But somehow I can't seem to keep the costs as low as most here. Maybe it's just where I am?! :)