Speaker cable length with monoblock amps?

$15k/$22.5k cables/amps proportion is not what I had in mind.
In any case, as I suggested above, one must be able to tell the difference between 2' and longer cables. And the difference should be real not something bordering non-existant. It will also depend on speakers. There is no general answer. If someone is concerned with resale, yeah, not shorter than 6', if not - listen. And if auditioning is not possible and there is no-one to advise in the paricular case - 6' is probably most reasonable. Personally, I never really think of the resale value. I upgrade rarely and if after ten years I'll have to throw away an amp I paid $5k for - so be it, that's less than $50 a month. I think about performance and try to get the most of it for the least money, after that business is over and listening to music is in order.

What price levels of monoblocks and cables are we talking about? At high level you should not worry too much about resale value.

This could be another test for audiophiles. Can you hear a difference between 2 and 3 feet speaker cables? How about 2 feet and 2 meters?
I suggest you first choose the cable brand, they are not created equal, just like people. Generally speaking, I would go with 3 or 5 feet depending on where binding posts are located.