I've tried a bunch of the arguably affordable mains cables, chord, monster, Russ andrews and so on. I'd say the RA cables had more of a negative effect, in fact my mains conditioners manual strongly advises against the woven types. (Maybe to help promote their own?)
The only cable which truly makes a small but noticeable difference are the QED XT5. They have a ferrite infused insulator running the whole length of the cable. Being a radio ham I've used ferrite to solve all sorts of problems so I was intrigued. I've dicected one to make a neater length to my TT. The ferrite is indeed present.
The little difference they made was a small but repeatable reduction to the noise floor. But we're talking cranking the volume with nothing playing and hearing the reduced hiss. Bear in mind also this is a purely analogue TT setup, maybe no effect with digital?
In terms of sound improvemet it's harder to judge. Maybe slightly more effortless and free flowing in the overall presentation. I figure the extra ferrite is reducing the ambient EMF which could be picked up by my MC cart (Sumiko Starling). Either way I'm happy with the slightly reduced noise.
Maybe the mega money cables are on another level but £150-200 is the top end I'd pay for a cable mains or otherwise.