I was looking at V-Caps, copper clad, North of $100 per cap!!!
Copper foil rolled with sheets of film plastic costs more than sputtering aluminum on plastic. Production quantities are probably flea-bite scale compared to any sensibly-priced coupling cap. There's undoubtedly a healthy profit margin added, but those caps would always be expensive due to pricy raw material and total lack of economy of scale. So I'm not exactly put off by the price.
I just don't believe you'll hear an improvement that justifies the cost
in a guitar amp. Maybe in hi-fi audio if everything else in the chain is as near-perfect as it can be.
The first 5E3 I built has "audiophile resistors" and copper-foil paper-in-oil caps. Know what it sounds like? A tweed Deluxe... I think after that episode I've paid for the right to the opinion that circuit architecture is a much greater factor in determining the amp's sound, though I can & do hear the differences between varieties of some parts.
Can't knock someone who trying to make a buck in the tube business.
Agreed. My real point (not really articulated) is it might be a mistake to assume Broskie endorses everything that appears in ads on his site. He might, but if I read into his writing correctly he's more likely to mean, "this is a good product if you have an application which requires it," rather than, "you must have this or your amp is garbage."