cheapest would have to be running it on the computer you made this post from .. assuming its always on and the delta to your power bill is negligible. as others

cheapest would have to be running it on the computer you made this post from .. assuming its always on and the delta to your power bill is negligible. as others have suggested if you have an RPi or some other low power SBC laying around, that would be ideal. i'm sure the major cloud providers have cheap / free tiers for this kind of work, but quite frankly i've been burned by run away pricing tiers too many times to ever consider using cloud again for a personal project .. so unless this is getting funded by a client try to host your own.