The point of diminishing returns goes by so many different names. I'm not going to delve too deeply into this subject, but I thought it makes for a little brain food. In short, it's an point on the economic scale of where a product's cost and value start to slow down.
We'll use Nikon cameras as an example as I know these best.
The Nikon D3500 will be where we start and can be bought for £359 with a kit lens. (I had to double check that because that seems insanely cheap!)
We'll move now to a semi professional aps-c camera; Nikon D7500. The Nikon D7500 can be bought for around £999, body only. (No lens included). The £650 price increase for a camera with no lens seems pretty huge. But, the difference in these cameras is ridiculously huge too. The D7500 is a far superior camera.
Next we'll move to full frame; Nikon D750. In comparison the D750 is older, but still relevant. The Nikon D750 can be had for around £1600 body only. Another £600 price increase, but the step up in camera isn't as big. The D750 is better (debatable) than the D7500, but not as significantly so as the D7500 over D3500.
Up another step to the Nikon D850. What an absolute beast of a camera! Probably the best camera Nikon has ever made! This can be yours for around £2300! Another £700 price increase. Again, a better camera than the D750, but not as much as a difference as the D3500 and D7500.
Lastly comes the Nikon D5. The price point now changes to a cool £5699! That's a whopping £3400 more than the D850. Being 2.5x more expensive than the D850 you may be thinking it's 2.5x the camera.. You'd be wrong.. Firstly, it's a different camera in practice. But, however you look at it, it's probably 20% better than the D850 is.
So to recap, the D7500 is around £600 more than the D3500 and twice the camera, at least.
The D750 is around £600 more than the D7500 and is around 60% better.
The D850 is around £700 more than the D750 and 50% better.
Lastly, the D5 is around a whopping £3400 more than a D850 and is probably 20% better.
We have to decide on whether to spend the extra money on what is only a slight increase of performance. Sometimes this isn’t justifiable, and that’s fine.