I’m relatively new to cycling (2 years), but have bought 3 bikes in that time, each one more expensive than the previous one. Here’s my take on the price/quality ratio.

The three bikes are:

Trek Crossrip Elite: AU$1500 (MSRP AU$1800)
Giant TCX Advance Pro 2: AU$2800 (MSRP $3500)
Specialized Roubaix Expert UDi2: AU$4500 (MSRP $6000)

The Giant is my daily commuter and the Spec Roubaix is my weekend bike

The Giant was a very noticeable upgrade to the Trek for the following reasons:
1) Much better ride quality - more compliant (less vibration), but more responsive
2) Less flex under load - the Trek was nowhere near as stiff when applying power
3) Much better gears - SRAM Rival vs Shimano Sora - I had lots of indexing issues with the Sora. The SRAM groupset hasn’t missed a beat.
4) Better brakes - SRAM hydro disks vs mechanical discs. I upgraded the Trek to TRP Hy-RD, but there still weren’t as good.
5) Better wheels & tires. I also upgraded the Giant’s wheels to Fulcrum Racing 5, but the stock PX2 were pretty good too (now my gravel wheels)
6) A lot lighter (c. 9.2kg vs 11.5kg) without pedals or accessories.

The Giant just feels a lot more responsive and engaging to ride. It was easily worth the money in my opinion.

However the step up from c. $3000 to $4500 was much less obvious.

The Roubaix has some nice features like the FutureShock suspension and Di2 gears, and nicer finishing kit, but it’s not hugely better.
I) The Shimano Ultegra Di2 is smoother than SRAM Rival
ii) The Shimano hydro disc-brakes are excellent, and better than the SRAM
iii) The DT Swiss wheels are better than the Giant PX2, but don’t feel very different to the Fulcrums (both alloy wheels)
iv) The frame is maybe a bit better, and is lighter, but I can’t feel a big difference.
v) It feels faster and more responsive (mostly geometry) - but in reality I go at about the same speed on both bikes.

I don’t regret getting the Roubaix, but it only feels like a small improvement over the Giant - despite the big uptick in price.

My experience is that diminishing returns probably set in at about US$2000-2500, and that above about $3000 you are seeing smaller improvements that become a personal value judgement.

I’d love a super aero bike or a featherweight climbing bike, but the reality is that my ability wouldn’t take advantage of these, and it would be hard to justify the cost for probably very little real benefit other than pride of ownership - which shouldn’t be dismissed as a valid reason.