I had this exact thought the other day when I was stood in the Specialized concept store looking at a limited edition Campag Super Record equipped beast sporting zipp 404s with an £11k price tag swinging from its neck.
£11k!
My carbon bike cost me £1k initially and I've probably spent at least another £1k on upgrades.
Is that bike between 5 to 10 times faster than my bike? Is it 5 to 10 times more desirable?
Is it good value for money?
I would hope it would be faster, but more to do with the wheels and possibly a small weight saving, but I imagine it would be marginal. In terms of group set you're comparing the top end Campag group to the top end SRAM group that I have, so there shouldn't be any significant shifting difference.
Frame would be lighter and now a little more modern but both carbon, both relatively aero, and with a similar geometry.
Now at the other end, would the difference between a £100 - £200 road bike be similarly marginal in comparison to my carbon bike?
I would think massively different with regards to weight, aerodynamics, research and development, frame material, shifting performance, comfort, rolling resistance on the hubs etc
There is certainly a particular price point where you can get a lightweight road bike that performs in a similar manner to the top end machines and you will only see very marginal improvements by switching to the very top end specs.
I doubt very much that if there is a chap in my club who always beats me to the top of long, hard, climbs on a bike the same spec as mine by say a minute, but then i spend £11k on this specialized, I'm not going to suddenly start beating him. But put him on a £100-£200 bike and I fancy my chances.
Regardless, it depends on what you want from your riding. A club rider who is doing a bit of racing will appreciate marginal gains. It helps to get a pb in a time trial or shave seconds off a sprint.
If you're just riding for fitness or enjoyment and the occasional sportive then the more you spend over around £800-£1500 you're probably not going to get value for money performance gains.