For myself, it would at least have to be at the point at which the core elements of the system - amp, pre-amp, sources - could not benefit more from an outlay similar to that of the cable - so, around 1.5K. But also, if an extra 1.5K would put me in a position to undertake a more substantive upgrade of one of these aspects then I'd probably do that and forgo the cable. That is, I'd really have to be confident that pretty much everything else was properly sorted, and that an outlay of this sort was be proportionate to funds allocated elsewhere - I can't imagine ever being in this position, and to be honest I don't really think I'd consider spending this much on a mains cable even if I was in a position to - I'd be more likely to explore hardwiring.

Ditto

What they said.

Straight answer to your actual question -  when whatever I was attaching it to cost at least 15 to 20 times as much as the lead itself.  Anything less and it seems a bit like the boy racers shoe-horning two grands worth of stereo into seven hundred quid's worth of car to my mind.

Also, could you settle for one lead?  Where would it go?  A power amp, what about he other one(s)? A  source component, what about the amps, and the pre if it's not passive and so on..?  None of this is ever going to happen for me because I'm never likely to have a six figure system, which is roughly what I'd be looking at to even start to convince myself a set of £1000+ mains leads would be a justifiable upgrade...although, personally, I don't think paying that much for a mains lead would ever be that much of a benefit.  Mains regen would probably come first for me and/or balanced mains.  After that hard-wiring.  Expensive kettle leads would be a long way down my priority list.

I'll differ from everyone else here. My Coherent 6D cable runs are of that order of magnitude in price and I can easily hear what they do, so at the level of my system they make sense. For a lesser system, I would spend on good decent mains cables but not that kind of money. It's all got to be in balance, no point going large in one area if you have a bottleneck elsewhere.

And as SQ said, a BMU or regenerator should come first as you will easily hear what it can do.

I've thrown all ideas of proportionality out of the window these days, having had my preconceptions challenged by a fab £500 plus interconnect in a modest system.

I'd simply say try as much stuff as you can. If you can get a loan of said item or any other, snap it up. You can't lose by trying.

* If it works for you and you can afford it, keep it.
*If it works and you can't afford it, at least you have something to aim for in terms of sound and perhaps a later purchase.
* If it doesn't work for you, then you can put it to bed for now.

These days, mains leads don't seem to make much difference for me, but I've had systems and homes where they have, so I try to keep an open mind.

I don't think the pricing represents proportional improvement - the difference between a £40 mains cable and a £600 mains cable,to me,isn't enough to justify the expenditure.
Each to their own though.

Maybe something like this could be part of a loan scheme similar to NVA where people can find out for themselves

If you follow the link that you provided, there's actually a pretty extensive write-up on the build of the cable, and on what - as you say - makes it special.

Maybe he'll start a loan scheme and let people borrow such products and see for themselves?