Can I press you on why the words ridiculous and absurd come to mind? I'm not arguing or even saying you're wrong - these are all just opinions of course - but I'm curious to know where you're coming from.
In my mind, saying a cheap amp does the job or the audience can't tell the difference is irrelevant to the argument. Your neighbour isn't affected by what you drive and how it performs while getting you to work after all. It's also unlikely a CS Les Paul will give the audience a more pleasurable evening over cheap knock off. This items luxury items for the user to enjoy, even if they are 'tools'.
A smart phone probably retails for 50 times its cost of manufacture. Cars, food, furniture, guitars... probably most things have a much higher comparative profit margin. Amps are very expensive to make unless you cheap out on components, or makes 1000's of them.
I guess what I'm saying is there isn't a huge difference between an Asian amp with the cheapest parts money can buy, and the top of the heap.
I suppose it's all in the eye of the beholder, what counts as absurd, and I don't have all the data to really say what the cost per amp is to produce some of these things which is the piece of info I need to make a more informed statement. So just to be clear I'd happily change my tune if someone could provide verifiable evidence of the costs to design/market/produce/distribute amplifiers compared to the sale price etc.
I'm also not saying it's easy to just slap together a circuit design and sell amps for a huge profit. Obviously if it were that easy, everyone would be doing it, shoot I'd be doing it too.
But *especially* amps like the new Mesa Mk VII which are basically the same circuit they already designed decades ago, markets itself now, and they have simply tacked on some new switching options/values... I'm not saying it's a bad amp, but the ROI on that original R&D sure has served them well. Even with US assembly, PCB tracing, cabinet building and other labor (support, repairs, distributing etc), at least *some* % of parts are imported (and there is nothing wrong with that), they must make out pretty well per unit.
What I'm getting at is, the profit margins on an amp like the new $3500 Mk VII must be really good. I think what
could be 10x the cost to build/distribute something counts as "absurd" markup. On the other hand, if I owned a business stake in an amp company, I'd probably argue that 10x markup is pretty "normal." I'm just using 10x as an example, I have no idea what the cost is to build one, and we never will because Mesa is never going to share that information (because likely it would make them look bad, even if it's just the industry average markup). Guitar players have a weird habit of paying more for non-quantitative things - vibes, "mojo," etc and that lets the prices and marketing run wild too. Each individual buyer will have their threshold for what they consider to be worth an asking price, and whether we are conscious of it or not, most people factor in how much something costs to produce into that.
Your smart phone analogy is way off by the way. A recent iPhone for example costs roughly $400-500 to manufacture and distribute, and retails for about $1000. That's only a bit over 2x markup. An amp is much simpler electronically and uses cheaper components than a phone does. Companies like Apple can afford to sell at lower markup like that because they are moving millions of units every day, as opposed to a higher end amp maker who might move a few per month (or worse).
Long story short:
You and I are both intense enthusiasts of this sort of thing so our threshold for spending is way above average, and that's probably dulled our senses to what the average musician considers to be a big deal in the grand scheme. I mean, I'll blow $2k on an amp no problem. So 4-6k like you said in the grand scheme of things, yeah, it's a HUGE amount of money. Just because a cheaper amp could do it isn't relevant when talking about tone, these are luxury items (it is art in general after all, and art/music is a luxury itself), but it is relevant to when people consider spending money, if something less expensive can do it "well enough." To relate to your car analogy, there will always be someone willing to spend that big money on a Ferrari, but there's a reason why Corvettes that perform nearly the same fly off of lots (here in the US at least) for 1/5 the price of the cheapest Ferrari.