On-line shopping makes this a little harder, I think, because in the "real" store you can touch, feel, see before you purchase and if you go to different stores and departments within a store, it becomes easier to compare items literally side-by-side, or at least via recent memory. Plus, it can be harder to envision paying a larger price on-line knowing it may not even fit, or you might be tempted to keep it rather than have the hassle of returning it. So, factoring in the psychology of your shopping style and options may help you decipher how best to upgrade your wardrobe.
Remember that "quality" usually has severa/ dimensions, but the minimum to me is fit! (or tailorability), then fabric and construction. Meaning, if it doesn't or can't fit, it won't matter whether it's made out of $500/yard fabric. So you can be analyzing various elements of your clothing as to what makes them look really great, and you will find different requirements for different items--the example above is great, that you might find a great white T shirt at Gap or K-mart; if you're not a hard to fit shape or size, there may be black pencil skirts at a range of price points, and the black fabric may be more forgiving of various, so you may not have to spend big $$ there; a wool coat though ought to feel luxurious, not scratchy, fit well, and so on.
If you are able to shop in person, I found it helpful to wear nicer clothes while shopping. That's like the advice from rute, but taking it a step further--wear something really good quality while you're actually shopping; it is an immediate reminder of your goals, it forms a basis for comparison, and it can help you say, no, this item does not match my current quality goal, but I know that there's better stuff out there, because I'm wearing some of it.
Read Angie's advice on the 3 P's (PPP) for trying to hold to a standard.