Cheaper = worse was a generalisation, my fault. My point was that the manufacturer does not include thicker/better shielded/blessed by the shaman cables with their less expensive models. There are some inexpensive cables with decent parameters. I can only comment on the quality of the cables I used/replaced, and subjectively I heard a difference in two out of two headphones recabled, but placebo never sleeps.
As for measuring differences, as I've mentioned before, among other things cables have their capacity, and in datasheets of many audio opamps you can see how do those chips fare driving capacitive loads of x or y pF.
Here is an example taken from opa227 pdf:
So there is a measureable difference in how will the opamp perform with driving cables of different capacity. Sure, not every opamp in every circuit will react the same but in some there surely can be a measurable difference. Whether it's audible or not, well, that's a subjective thing.
Cables can aslo act as filters, and while they are still just copper their build can suppress some interference. Below is a quote from the cable catalog of KLOTZ, a german cable manufacuter:
In audio applications, StarQuad cables are primarily used for highamplification
microphone lines if powerful electromagnetic fields
occur in their surrounding area, generated by e.g. lighting control
equipment, power lines, transformers, transmitter masts and so on.
Microphone lines are particularly sensitive, because their signal
amplifications can extend up to 70 dB so that any interference effect
to the cable will be amplified in the same way.
The name StarQuad refers to the cable design, featuring 4-core
stranding, with opposite cores forming pairs to create a balanced
line. This kind of cable is also used as a compact data cable in
Design and function:
The cable's outstanding interference reduction properties (low noise
pick-up) are created by the use of external wiring connecting the
opposite cores. The result is a parallel connection of two balanced
lines twisted together, geometrically halving the length of twist
compared to a single pair. The shorter the length of twist of a
balanced line, the more effective its suppression of interference.
So high capacitance cables can cause overshoots or oscillation in some systems, and there are different types of noise and interference reduction, each suited for different applications. Again, whether you hear it or not is totally subjective but there are means to scientifically differentiate cables of different constructions and materials (like higher conductivity of silver over copper). Or when a given cable is too thin to deliver enough current, same thing happens- it plays a role and can audible.
Saying that ALL cables sound alike is a generalisation, and is false. Some setups may require that the cable is of low capacity/inductance or twisted this or that way. Sure, by the time we reach high-end cables territory all the basic requirements are probably fulfilled and some magic (marketing) starts to take place, although I have never heard high-end cable on an accordingly expensive system, so that's just my guess.