I don't really have a monthly clothing budget, but when I need something, I buy it. I don't go shopping just for fun, or to "browse." I would say that I buy 2 new pairs of pants every year...nice quality, Prana or Patagonia, and they last me for a few years. I tend to be super hard on clothing, and so I feel like I'm wearing through pants every 2-3 years. I don't buy many "cute" shirts/skirts/dresses...and if I do it's at the local consignment shop where they have great lightly used brands that I love.
I spend a lot of time outdoors, skiing, hiking, mountain biking, etc...and live in a very cold mountain town, so I have a ton of outerwear that would probably be seen as extravagant on this forum. I use every piece of it, and I'm never bummed that I spent $300 on a down jacket. I also work in an industry where I get a lot of deals and very expensive outdoor brands, so I try to buy the nicest thing I can afford. Even so, when you ski 60+ days a year, bike 250+ days a year, and hike the remaining days, stuff wears out fast.
So I would say every year I estimate I spend about $1000 between a new jacket, or pants, or a couple new pairs of riding shorts, hiking shoes, base layers, running shoes, tights, etc...I also sell older stuff that I no longer use that is in good shape, or that I bought and realized wasn't what I needed. I would estimate that half of what I spend on outdoor apparel/clothing is money that I made selling older, used gear.