It really depends on a hardware.
E.g no point to upgrade CPU if the average diff is less that 20%. Since it just would be waste of money.
GPU wise, diff of ~45% can be quite substantial. For example: I had GTX 1060 3GB GPU, which i upgraded to GTX 1660 Ti 6GB,
Average diff between the two is ~45%. In reality, at 1080p, GTX 1060 was able to get 60-80 FPS, while GTX 1660 Ti gets me 120-144 FPS. So, considerable upgrade.
RAM wise, average diff of ~25% can be considered as good upgrade, namely when RAM frequency is considerably increased. E.g 2x 8GB 2133 Mhz vs 2x 8GB 3200 Mhz,
But if RAM frequency upgrade would be less, e.g from 2400/2666 Mhz to 3200 Mhz, then buying new RAM kit, just for faster frequency, IMO, isn't worth it. But if you'd also upgrade RAM amount as well, e.g 2x 8GB 2666 Mhz to 2x 16GB 3200 Mhz, then it would be worthwhile.
The most difficult part on all of this is, how much the performance increase is costing. And here, rather than looking at upgrade cost alone, i'd suggest to look at the value of specific part. Value is price to performance ratio.
Here, i'll take desktop RTX 3060 and 3070 as an example (since laptop GPUs aren't sold individually, thus, can't be accurately valued).
Cheapest RTX 3060 costs currently ~$360, while cheapest RTX 3070 costs ~$635. The price diff isn't double but it's quite high, ~$275. Now, we know that RTX 3070 is ~55% better than RTX 3060, so is RTX 3070 worth the extra 275 over RTX 3060?
I'd say no, due to one critical fact. Namely, all RTX 3070 GPUs come with 8GB of VRAM. While there are RTX 3060 GPUs with 12GB of VRAM (like the one i included in the pcpp example). So, besides paying more, you'll also loose on the VRAM amount. Thus, IMO, RTX 3060 has higher value than RTX 3070.