ill chime in relative to aluminum frames, because they seem to run the whole range of prices and quality, and i've had experience with a cross section of them.

in terms of the raw frame, quality is determined by 3 things more or less.

alloy: these can be standard 6000 or 7000 series alloys at the low-mid range, you pay more for scandium alloys that allow for lighter frames.

tubing: how it is shaped or "butted" low end-frames will have little or no butting meaning the tube has the same cross section along its whole length. this makes the bike heavier and often less stiff. heavily, or continuously butted tubes (like on caad frames) use only as much material as needed at any point to achieve the best strength/weight ratio. butted tubes are harder to produce and thus cost more.

welds: low-end welds will look lumpy and small, high quality welds will look like big, completely smooth fillets. these are the strongest and least likely to crack. low-end welds can be done by robots, but high end welds will be done by a skilled welder who knows just how much material too add (thus it costs more)

even high end manufactures like trek and giant will have frames with many of the lower end features described above, but they will come with a good warranty, and thus demand a higher price than an otherwise similar frame bought on line without a warranty or as much of a rep.

also consider that most low end aluminum frames (from all brands) are made in a handful of chinease/Taiwanese factories. so the only difference between a large sector of that market can be just geometry and brand sticker.