I wonder what the cheapest possible build for something like this is, if you loosen enough constraints - let's say, if we allow it to be externally powered, and

I wonder what the cheapest possible build for something like this is, if you loosen enough constraints - let's say, if we allow it to be externally powered, and agree that size doesn't matter. Would it be cheaper (and/or safer) to build a larger one, allowing for larger-but-slower flywheels? Could you make one out of a couple of scrap bicycles?