I can remember buying an i5-4690K six years ago (4 cores / 4 threads) for close to $200. 16GB of ram cost twice as much and you couldn't even buy a 500 GB NVME drive then. A 500 GB Sata drive cost well over $120 then.
About this item/ the i5-10400
6 Cores / 12 Threads
Socket type LGA 1200
Up to 4.3 GHz
Compatible with Intel 400 series chipset based motherboards
We may be closer to the end of Intel support from macOS but I wouldn't hesitate to build now and use it for the next 4-6 years. Windows and Linux are still going to install and run fine on this hardware even 7-8 years from now.
Looks like Intel has slashed prices yet again across the board - the Core i5-10400 is now back at $160 whilst the rest of the range is also lower. You can now get a i7-10700F for $280 while the i9-10900K is now available for $468.