i think you will lose in the end this way if you actually factor in power costs.
Yup, 6x old i5 thinkpads will require more space, more power and will likely be more expensive than 6x Pi.
I'd take 6 old ThinkPad over 6 new Pi any day. :)
I host a lot out of my house on a T430S with a cracked screen.
I know there are thinkpad hoarders with like 10 or 20+ old thinkpads. Seems to be a nice hobby, but you will rarely find one who builds a thinkpad cluster.
In the case of Raspberry Pi it is pretty normal nowadays to connect a bunch of them into a cluster and have some fun. In fact, it has become so popular, that you u can find stackable cluster cases and other Pi cluster hardware on amazon.
Not when you factor in performance per watt.
When you're done with the cluster, you can likely sell them for nearly what you paid for them.