No matter what you're wearing, you can always look like you payed a lot for it.
Make sure they're a good fit.
It doesn't matter what you're wearing, it should always fit. This doesn't mean it has to perfectly fir your body, it just has to look deliberate. For example, if you're trying to wear something oversized, make sure it is actually oversized. If something is too big and it isn't supposed to be, cinch it in with a belt or tuck it into some high waisted pants. Notch or cuff the bottom of your jeans that are too long or roll your sleeves up to a three quarter or even above the elbow length. Your clothes don't have to be the exact size you wear, especially if they were thrifted or reworked. I hardly ever wear clothes that I picked out brand new in my size. Just make sure however the clothes fit, it looks like it was on purpose and works with the rest of the outfit.
Stop being so boring.
If you're wearing the same thing as everyone else, nobody is going to remember your outfit for being special. Part of looking expensive is embracing that high end brands often sell bold and quirky pieces and incorporating that into your own style. If the items you wear don't look like something you could buy in any old store, they're likely to look more exclusive and therefore expensive. So many people that are famous for their style dress in weird and unconventional ways, but that's what they're known for. Find something quirky that you love and embrace it fully.
Lay off the accessories.
I'm not saying you should completely forget about your accessories, just wear them more intentionally. You don't have to layer on every designer piece of designer (or fake designer, we're all guilty) jewelry you own at once. Choose one or two pieces that go the best with the outfit. It looks so much better when your outfit is simple yet cohesive instead of looking cluttered. (unless you're going for that layered chain necklace look that's big right now, in that case give it a try.) If you wear all of your nice jewelry at once, it looks like you're trying to show it all off, which is not something that makes your look appear expensive.
Focus on the fabric.
This one is really important to me. The quality and finish of the fabric is in my opinion the biggest thing that makes it look high end or not. Fabrics that are, or even appear to be more durable look the best. For example, most of the silk fabrics I would buy would not be real silk, but some kind of poly satin or imitation silk. If I'm buying anything like this, I try to find the items with fabric that most closely resembles real silk. This means its not too shiny, as some fake silks have too much sheen to them that makes them look like plastic. It also needs to be thick enough that it isn't see through. Other than silk, denim is the most important to me in this sense. I like denims that are 100% cotton, or are as close to that as possible. Personally I don't like denims that have a lot of stretch to them, so I always choose ones that don't have any. I like denim finishes that are very matte. I find that some denims (usually that have high polyester content) look a little shiny. Choose fabrics that look and feel nice. Sometimes a piece will have a stunning cut or design, but the fabric is so bad I don't want to buy it. Sometimes the fabric of a garment elevates a design that isn't anything special to a really beautiful piece.
Be subtle with your logos.
Don't wear six different brand logos all at once. Be mindful of your logos. Some brands are known for their flashy logos, and in that case I would choose one logo piece to wear and go with more plain items for the rest of the outfit. I find that the outfits that grab my attention the most are often ones with no logos at all. It makes me wonder where that person got their clothes from.
Try to match your metals.
This one is a little harder to do, but makes an outfit look really put together. Making sure that all of the metals that you're wearing are the same color can elevate your look. For example, the hardware on your bag, the buckle on your belt, and your jewelry would all match. Like I said, this is hard to do because you're not always going to have everything in silver or everything in gold, but if you can pull it off it's going to make you look really good and show that you put effort into your look.