You have probably walked into a room that felt expensive and been unable to say why. Nothing in it was obviously luxurious. It just read as considered.
Having looked at a great many of these rooms, the pattern is consistent and it has very little to do with what anything cost. It comes down to four things, and all of them are free to apply.
1. Texture is doing the work colour usually gets credit for
Rooms that feel flat are usually rooms where everything has the same surface. Smooth painted walls, a smooth sofa, a smooth table, a flat rug. The colours can be perfect and it will still look like a catalogue page.
Rooms that feel rich have contrast in how things feel, not just how they look. Something woven against something smooth. Something matt against something with a sheen. Rough ceramic next to polished glass. A chunky knit throw on a flat weave sofa.
The useful test is to photograph the room in black and white. Colour disappears and you see only texture and tone. If the black and white photo looks boring, the room is relying on colour to do a job texture should be doing.
2. There is space around things
This is the one that costs nothing and is hardest to do. Expensive looking rooms have empty surfaces. A sideboard with three objects on it and a lot of bare wood reads far better than the same sideboard with eleven objects on it, even if every object is lovely.
Space around an object tells the eye the object was chosen. Objects crammed together tell the eye they accumulated. That is the entire difference and it is why editing a shelf makes more difference than restyling it.
3. Nothing is floating at a random height
Art hung too high is the single most common thing that makes a decent room look amateur, and almost everyone does it. The centre of a picture should sit at roughly eye level, about 145 to 150 centimetres from the floor. If it is above a sofa, the bottom edge should sit around 15 to 20 centimetres above the back of it, so the two read as one group rather than two unrelated things.
The same principle applies to pendant lights over a table, curtain poles, and mirrors. Things that relate to each other should look related. Curtains in particular: hung close to the top of the window frame they make a room look shorter, hung high and wide they make the window look larger and the ceiling higher.
4. The light is warm and there is not one source of it
Cool white overhead lighting will flatten any room regardless of what is in it. It is the lighting of an office, and your brain knows.
Warm bulbs, around 2700 kelvin, and at least three separate sources at different heights. That is genuinely most of the effect. Two lamps and a dimmer will do more for how a room feels than a new sofa, for a fraction of the money.
The uncomfortable bit
Most of what makes a room feel expensive involves taking things away, not adding them. Fewer objects with more space around them. Fewer colours repeated more often. Less overhead light and more low warm light.
Which means the cheapest possible improvement to most rooms is an evening, a box, and a willingness to put a third of what is on display somewhere else for a fortnight to see whether you miss it. Usually you do not.