i always envisioned that if i ever did this, i would set the spot price to the equivalent of infinity/max and then monitor it myself, terminating myself if i determined that continuing to run at the spot price was more expensive.

why? sure, you don't want to pay more than the on demand price, but iirc spot prices often spike very momentarily. so the question becomes whether the sum over spike time cost at the higher spot price exceeds the time to boot/shutdown/migration overhead at the on demand price.

but i've never actually tried it so...