I don't think you could justify spending that amount of money on a bike unless it is one of your life's deepest passions. Which seems rare.
Does the quality of bikes really change that much when you go up the price range, for amateur users?
But the "amateur using pro gear" is widespread, not just in cycling. And what's acceptable is a matter of culture. I've noticed that a person riding a $4k bike will get criticized by someone who drives a $40k car, with no irony.
I'm an amateur musician, but I play a professional quality instrument. It really comes down to the sheer enjoyment of it. My instrument won't get me into the Chicago Symphony, but it sounds nice in my living room. And it still cost less than many pleasures that people afford themselves.
At first glance, the wooden bike has to be for someone who is interested in the aesthetics and novelty of it.
Disclaimer: I ride a bike that I assembled from mostly spare parts for less than $100.
If you want specific solutions things often get expensive. And yes there is a world of differens from a €1000 bike to one that costs €4000.
For most riders, going above an entry level carbon fiber bike with Shimano 105 components and alloy rims is probably overkill. That price point is between $1500 and $2500US, depending on manufacturer and other options.
If you're a larger rider, getting a better grade of carbon frame is worth it. I'm about 98kg, so I took that option. My bike was $3500US, but still with alloy rims.
Wheels matter a LOT. The next step up in the Specialized line that year was exactly the same in all respects except wheels (carbon, aero) and price ($5,000US).
The other thing you see in nicer enthusiast bikes is electronic shifting. This adds, say, $800 to $1500 to the price vs. an equivalent cable-pull bike. It sounds insane, but it's really quite nice -- and never goes out of adjustment.
Disc brakes are taking over as well, but still command a slight premium. I don't have them because I live in a flat place, but if I lived in northern California or Colorado, they'd be a no-brainer -- they work WAY better on long descents or in the wet.
Oh, and if you want titanium instead of carbon, well, the whole game STARTS much higher. The most common ti frames I see are from a company called Moots; entry level for the frame alone is about $4000US -- more than my bike was originally.
Obviously, I don't see ti bikes except under fairly well-heeled riders MOST of the time (there are exceptions) -- but I see lots of normal middle class folks on $2500-$4000 (retail) bikes. That's kind of the default in the groups I ride in. Going higher is possible, but of limited utility. Going lower means you have a bit less fun, honestly. They didn't all pay full price or buy new, but that's the grade of bike around me on most rides.
Even the Tiagra line is very good these days. Aside from the 11th cog on the 105's, I'd say that they're pretty even in terms of performance. Especially in regards to shifting.
The maxim in the cycling community where I live is that 105 is fine, but Ultegra is worth it if you can afford it, and Dura Ace is just showing off. I think that's pretty accurate.
(Of course, it probably needs updating for Di2/eTap...)
I don't think you've spent a lot of time around bikers!
“full spec” is $16k.