For a while I was thinking of spinning up an AWS Free instance, until I spotted this addition to their terms :

I don't think Amazon AWS is opposed to crypto-mining, as it is opposed to tasks using 100% CPU load for 100% of the time. That doesn't scale well in a shared resource environment.

We mostly use discarded hardware that we used to sell off for pennies*. As corporate power tariffs are tax free above a certain usage tier in our country, we pay extremely little for our power usage. So using hardware we already own, combined with cheap power, is in our case a viable alternative to buying cloud instances. Cloud remains an expensive choice when you need to run your tasks at full load for 24/7 (that's why mainframes are still being sold today). My blogpost about our setup might be intersting to some of you.

* A lot of corporates (at least the ones that operate their own datacenter) use this process when it comes to selling off disused or outdated hardware. Such corporates might be willing to sell their old hardware to you if you either pay a slight premium over 'the other guys' or when you can convince them about the humanitarian aspect the hardware will end up being used for.