Free yes. Power efficient no. Unless you switch your laptops every two years, it's unlikely to be more efficient.
some benchmarks show the Raspberry Pi 4 idling below 3W and consuming a tad over 6W under sustained high load.
Power consumption is not an argument that's in favor of old laptops.
That is the key. The RPi works for idling, but anything else gets throttled pretty bad. I used to self host on the RPi, but it was just not enough[1]. Laptops/mini-PCs will have a much better burstable-to-idle power ratio (6/3W vs 35/8W).
1: https://www.kassner.com.br/en/2022/03/16/update-to-my-zfs-ba...
I don't have a dog in this race, but I recall that RPi's throttling issues when subjected to high loads were actually thermal throttling. Meaning, you picked up a naked board and started blasting benchmarks until it overheated.
You cannot make sweeping statements about RPi's throttling while leaving out the root cause.
So that'd take 30 years to pay back. Or, with discounted cash flow applied... Probably never.