I didn't read the paper in depth, just skimmed it, but it seems like their assumption is that the astronauts will be eating some sort of microbe slurry; so I think that the cost includes only that, not using microbes to feed higher-order life or growing plants.
Agree, but the post I was responding to was suggesting it be used as an agricultural feedstock to feed things the astronauts eat - that’s where my order-of-magnitude calculation came from.
