There are lots of them out there.
For example passmark is one of many.
https://www.cpubenchmark.net/cpu_list.php
I just go find my CPU and use the number to compare to eBay listings. Is it perfect? No.
But it gives you an idea. Each site has some set of algorithms and they get a score for how quickly it can execute on that hardware.
Some of them they allow users to run their system and submit numbers so you get a better sense.
I found a used HP business small form factor i5 6th or 7th gen intel for very cheap, slapped a few SATA drives in the thing, and one of the M.2 Coral TPUs.
it is running 20-30% CPU load with 6 cameras on it - but they standard HD - I pumped one at 4k, and it loaded up much higher, so I scaled back to all 1080p or less. The TPU doesn’t even hit 1% from what i’ve seen. I should probably load a better TFLite model. Nothing mission critical - mostly a novelty.
not the most power efficient setup.